Dagworth-Granger, Hector (HBP ch. 9): Founder of the Most Extraordinary Society of Potioneers.
Etym: Couldn't find anything on Dagworth; see Granger for that etymology.
Dai Llewellyn Ward (OotP ch. 22): A ward for bite victims at St. Mungo's Hospital.
Etym: Named for "Dangerous" Dai Llewellyn, whoever he is.
Daily Prophet: A daily newspaper, the primary news source for most British wizards.
Damocles (Belby?) (HBP ch. 7): Marcus Belby's uncle, one of Horace Slughorn's former students, who went on to invent the Wolfsbane Potion.
Etym: A courtier in Greek myth who, to make a point about how precarious life is, found himself under the famous dangling sword.
dandelion juice (OotP ch. 38): Apparently a perfectly ordinary drink for wizards.
Daphne Greengrass: Etym: A nymph who escaped Apollo's advances by being turned into a laurel, which ever after was sacred to Apollo. From this came the use of the laurel as an award for achievement.
Dark Arts Outsmarted, The (OotP ch. 18): A book found in the Room of Requirement.
Dark Force Defense League: An organization of which Gilderoy Lockhart is an honorary member. It may be some sort of vigilante group.
Dark Forces, The: A Guide to Self-Protection: The first-year textbook for Defense Against the Dark Arts.
Davies, Roger (PoA ch. 14): Captain of the Ravenclaw Quidditch team.
Etym: From the Hebrew name David, meaning "beloved". Sarah Emily Davies (1830-1921) was a pioneer in the movement to allow women into colleges, and was the founder of Girton College, Cambridge.
Davy Gudgeon [David]:
Etym: Hebrew, meaning "beloved". St. David is the patron saint of Wales.
Dawlish (OotP ch. 27): An Auror who came to arrest Dumbledore in Harry's fifth year, and later was assigned to guard Hogwarts anyway.
Etym: Might be related to dawly, a dialect word in northern England meaning "miserable, gloomy, lonely".
Daydream Charm (HBP ch. 6): A product created by Weasley's Wizard Wheezes to make classes more pleasant.
Dean Thomas: Etym: From Middle English dene "valley", or deen, a borrowing of a word that ultimately meant "a leader of ten men".
Dearborn, Caradoc (OotP ch. 9): Etym: None found, but might just be from dear and born.
deathday: The date on which a ghost comes into being, or the anniversary of that date. Like a birthday, a cause for celebration.
Death Chamber (OotP ch. 35): The room in the Department of Mysteries where Sirius Black died.
Death Eaters: Voldemort's merry band of evildoers, now nearly all killed or captured.
Killed in the first war: Evan Rosier, Wilkes
Killed in Voldemort's service since: Bartemius Crouch junior, Bellatrix Lestrange
Either killed or captured: Alecto Carrow, Amycus Carrow, Avery, Crabbe senior, Antonin Dolohov, Gibbon, Fenrir Greyback, Goyle senior, Jugson, Rabastan Lestrange Rodolphus Lestrange, Walden Macnair, Lucius Malfoy, Mulciber, Nott senior, Rookwood, Travers, Yaxley
Whereabouts unknown: Ludo Bagman Renounced Voldemort (and killed) in the first war: Regulus Black
Renounced Voldemort (and killed) since: Igor Karkaroff, Peter Pettigrew (sorta), Severus Snape
Deathly Hallows (DH ch. 20): The Elder Wand, the Resurrection Stone, and the Cloak of Invisibility. If you have seen any of these items, please contact Death.
Death Omens: What to Do When You Know the Worst is Coming: A book in Flourish and Blotts.
Deathstick (DH ch. 21): A colloquial but much cooler name for the Elder Wand.
Decoy Detonator (HBP ch. 6): A Weasley's Wizard Wheezes product which provides a convenient nearby distraction when you need one.
Decree for Justifiable Confiscation (DH ch. 7): A law allowing the Ministry of Magic to, er, inspect a wizard's estate. Probably called that because it's quicker to say than "Decree for Helping Ourselves to Whatever Looks Interesting".
Decree for the Reasonable Restriction of Underage Wizardry: A law dating from 1875 which disallows student wizards from using magic outside of school.
Dedalus Diggle: Etym: Alternate spelling of Daedalus, of course.
Defense Against the Dark Arts: A required subject for all Hogwarts students, but by far the most hazardous class to teach. Teachers so far:
Harry's first year: Professor Quirrell
Second year: Gilderoy Lockhart
Third year: Remus Lupin (with a bit of substitute teaching from Professor Snape, nearly dodging the jinx)
Fourth year: Bartemius Crouch disguised as Alastor Moody
Fifth year: Dolores Umbridge
Sixth year: Snape again, unfortunately
Seventh year: Amycus Carrow
Defensive Magical Theory (OotP ch. 9): The textbook for Defense Against the Dark Arts in Harry's fifth year.
Deflagration Deluxe (OotP ch. 28): A collection of Weasleys' Wildfire Whiz-Bangs.
Deflating Draft: The antidote to a Swelling Solution.
Defodio (DH ch. 26): An incantation for tunnel-digging.
Etym: Latin, "I dig down", "cover", "bury", "conceal". (Just plain "dig" is fodio.)
Delacour, Apolline (DH ch. 6): Fleur and Gabrielle's mother.
Etym: See below.
Delacour Weasley, Fleur Isabelle: Etym: French for "of the court". A "flower of the court" would be a particularly striking noble lady.
Delacour, Gabrielle: Fleur Delacour's little sister.
Etym: See above.
Delacour, Monsieur (DH ch. 6): Fleur and Gabrielle's unnamed father.
Delaney-Podmore, Sir Patrick (CoS ch. 8): The head of the Headless Hunt.
Etym: Delaney may be from a French root meaning "wool" or "alder grove", or the Gaelic patronymic Ó Dubhshláine, composed of the elements dubh "black" + slán "challenge, defiance".
Podmore is of uncertain origin, possibly from Middle English pod or pad "frog" + more "fen, marsh".
Deletrius: An incantation to banish a summoned creature.
Etym: An alteration of deleterious, maybe.
Deluminator (DH ch. 7): Albus Dumbledore's name for his Put-Outer. (Or possibly the proper name all along, and it got changed for the US edition.)
Etym: It's the thing which causes things to be non-luminous.
Demelza Robins: Etym: The name comes from a placename, and was popularized by a character in the Poldark series.
dementor: A corpselike, possibly undead creature that feeds on emotions, draining away a person's will to live. Usually swathed in a cloak, their faces are seen only by people about to receive a dementor's kiss. Dementors are the guards of Azkaban.
Etym: Invention from demented, as in cause to be.
dementor's kiss: How a dementor sucks out a person's soul, leaving them in a permanent vegetative state.
Demiguise (DH ch. 21): A creature whose hair can be made into an invisibility cloak, although it will eventually wear out and remain visible. For more information, see Fantastic Beasts and Where to Find Them.
Etym: Demi- (half) + guise (appearance).
de Mimsy-Porpington: see Mimsy-Porpington.
Dennis (PS ch. 3): Part of Dudley Dursley's gang.
Etym: Derived from Dionysius, though the exact etymology is uncertain. St. Denis is a patron saint of France, who according to legend was martyred by decapitation. He is portrayed in art as a headless living figure.
Dennis Bishop: Etym: See above.
Dennis Creevey: Etym: See above.
Densaugeo: Incantation for a curse that makes the target's teeth grow.
Etym: Latin dens "tooth" + augeo "I augment". To make grammatical sense, ought to be dentemaugeo ("tooth" as direct object) or densauge (imperative: "tooth, grow!").
Department for the Regulation and Control of Magical Creatures: One of the many departments of the Ministry of Magic, incorporating the Beast Division, Being Division, Spirit Division, the Goblin Liaison Office, and the Pest Advisory Bureau.
Department of International Magical Cooperation: Another department of the Ministry of Magic, responsible in part for overseeing the Quidditch World Cup and the Triwizard Tournament, and containing the International Magical Trading Standards Body, the International Magical Office of Law, and the British seats of the International Confederation of Wizards.
Department of Magical Accidents and Catastrophes (OotP ch. 7): Another arm of the Ministry of Magic which holds the Accidental Magic Reversal Squad, the headquarters for Obliviators, and the Muggle-Worthy Excuse Committee.
Department of Magical Games and Sports: Yet another tentacle of the Ministry of Magic, incorporating the British and Irish Quidditch League, the Official Gobstones Club, and, for some reason, the Ludicrous Patents Office.
Department of Magical Law Enforcement (OotP ch. 7): A piece of the Ministry of Magic which contains the Magical Law Enforcement Squad, the Improper Use of Magic Office, the headquarters for the Aurors, and the administrative services for the Wizengamot.
Department of Magical Transport (OotP ch. 7): Another name for the Department of Magical Transportation.
Department of Magical Transportation: A section of the Ministry of Magic which contains the Floo Network Authority, Broom Regulatory Control, the Portkey Office, and the Apparation Test Center.
Department of Mysteries: The research arm of the Ministry of Magic.
Deprimo (DH ch. 21): An incantation that knocks a hole in the floor.
Etym: From Latin deprimere, "press down" or "sink down".
Derek (PoA ch. 11): A Hogwarts student, two years behind Harry, house unknown.
Derrick (PoA ch. 15): A Beater on the Slytherin Quidditch team.
Etym: Related to Theoderic, "people-rule". As a common noun, from the surname of a noted hangman at Tyburn, circa 1600.
Dervish and Banges: A toy shop in Hogsmeade.
Etym: Dervish is from Arabic darwiish, meaning a member of a Sufi fraternity, and is also used in fantasy to mean a whirling supernatural being. Banges would be an alteration of bangs.
Derwent, Dilys (OotP ch. 22): An 18th century Healer at St. Mungo's Hospital, after which a stint as headmistress of Hogwarts must have seemed fairly easy.
Descendo (DH ch. 31): An incantation to make something fall down.
Etym: Latin, "I descend". I guess it's supposed to be transitive?
Detachable Cribbing Cuffs (OotP ch. 31): A banned item at O.W.L. exams.
Deverill, Barnabas (DH ch. 21): A former owner of the Elder Wand, killed by Loxias for it.
Etym: Closest I could find was Everill, which goes back to an Old English name constructed from eofor "wild boar" + hild "battle".
Devil's Snare: A magical plant which grabs hold of anyone within range. (What it plans to do with them next has not been recorded.) It recoils from bright light.
Devon: A region of the UK which is home to the Flamels.
Diagon Alley: A wizard shopping district somewhere in London, accessible from The Leaky Cauldron and the Floo Network. Establishments therein include Eeylops Owl Emporium, Flourish and Blotts, Gringotts, Madam Malkin's Robes for All Occasions, Gambol and Japes, Florean Fortescue's ice cream parlor, Magical Menagerie, Quality Quidditch Supplies, Twilfitt and Tatting, Weasley's Wizard Wheezes, and Ollivander's.
Diffindo: The incantation for the Severing Charm.
Etym: Possibly Latin, "I split, cleave". Imperative: diffindere.
Diggle, Dedalus: A wizard who, according to Professor McGonagall, "never had much sense". Harry met him at The Leaky Cauldron.
Etym: Probably from the word dighel, meaning secret or obscure, though the proper etymology of the surname is different.
Diggory, Amos (GoF ch. 6): Cedric Diggory's father. He works for the Department for the Regulation and Control of Magical Creatures.
Diggory, Cedric: A Hufflepuff, two years ahead of Harry, the captain and Seeker of the Hufflepuff Quidditch team and one of the Hogwarts champions in the Triwizard Tournament. Also a co-winner of the tournament, which inevitably leads to him being killed seconds later. Now exists as a shade. Wand: 12.25", springy, ash and unicorn hair.
Etym: Goes back to the medieval romance of Sir Degaré, and is probably from French égaré "strayed, lost".
Dijon (OotP ch. 20): A region of France that Hagrid and Maxime passed through on their way to find the giants.
dilligrout (HBP ch. 12): A password used by the Fat Lady.
Etym: An actual obsolete word; it was pottage offered to the kings of England on their coronation day.
Dillonsby, Ivor (DH ch. 2): A wizard who claims Albus Dumbledore plagiarized his dragon's blood research.
Etym: Couldn't find any.
Dilys Derwent: Etym: None found.
Dimitrov (GoF ch. 8): A Chaser on the Bulgarian national Quidditch team.
Etym: From Greek Deemeetrios "(follower) of Demeter".
Dingle, Harold (OotP ch. 31): A student who was selling fake powdered dragon claw as an exam-taking aid.
Etym: From the Middle English word for a small wooded dell or hollow.
Dippet, Armando (CoS ch. 13): The headmaster of Hogwarts in Tom Riddle's time, probably Dumbledore's immediate predecessor.
Etym: Variation of Theobald, from the Germanic personal name *þeudo "people, race" + bald "bold, brave".
Dirigible Plum (DH ch. 20): A radishlike fruit which grows above ground. The name would suggest you can make it do something.
Dirk Cresswell: Etym: A Flemish version of Derrick.
Disapparation: Apparation as seen from the point of departure.
Disarming Charm: A spell that knocks an opponent's wand out of their grip. Incantation: Expelliarmus.
Disillusion (OotP ch. 3): To cast a Disillusionment Charm on.
Disillusionment Charm (OotP ch. 3): A spell that gives the target perfect camouflage.
Dissendium: The password to open the secret passage from Hogwarts to Honeydukes.
Etym: Latin, "dissension, discord".
dittany: OED lists a number of possible plants but, for our purposes, probably Origanum dictamnus aka Dictamnus creticus, once alleged to have medicinal virtues.
Divination: An elective subject at Hogwarts, taught, in a loose sense of the word, by Professor Trelawney.
Dobbs, Emma (GoF ch. 12): A Hogwarts student three years behind Harry, house unknown.
Etym: From a variation of Robert, which comes from Old English/German hrothi "fame" + berhta "bright".
Dobby: A house-elf formerly in the employ of Lucius Malfoy, who went to Hogwarts to break new ground with the unthinkable practice of being paid to work, and later to Shell Cottage to wind up getting ground broken for him.
Etym: Word for a household sprite or apparition, particularly a brownie. See house-elf for more.
Doge, Elphias (OotP ch. 3): A member of the Order of the Phoenix.
Etym: The title of the ruler of Venice from the 8th to 18th centuries, also used as the title of a civil official for a time in Genoa.
Dolohov, Antonin (GoF ch. 30): A Death Eater, imprisoned in Azkaban, freed, and sent right back after the Death Eater raid on the Ministry of Magic.
Etym: No info on the name and the nearest Russian words don't look too likely at the moment.
Dolores Jane Umbridge: Etym: Spanish for "sorrows".
Dorcas Meadowes: Etym: A Biblical figure whose name became associated with women's associations that made clothes to give to the poor.
Doreen Figg, Arabella: Etym: Probably an Irish version of Dorothy, "gift of God".
Doris Crockford: Etym: The name of a sea nymph in Greek myth, origin unknown. Noted as a given name only as far back as 1819.
Doris Purkiss: Etym: See above.
Dorkins, Mary (OotP ch. 1): A TV news reporter.
Etym: No info on the name. Used here, the closeness to dork might be intentional.
Dot [Dorothy] (GoF ch. 1): The cook at The Hanged Man.
Etym: The name is believed to have developed from an arbitrary inversion of Theodora, Greek for "God's gift".
Downing Street (DH ch. 5): The street on which the British prime minister lives; used as shorthand for that residence.
doxy (OotP ch. 6): Small, beetle-like magical creatures with a venemous bite.
Etym: Not sure.
Doxycide (OotP ch. 6): An aerosol spray that will paralyze a doxy.
Draco Malfoy: Etym: Latin for "dragon", though the direct source for Mr. Malfoy's name is probably the Athenian ruler who gave us the word draconian.
Dragomir Gorgovitch: Etym: From Old Slavic dorogo "dear, beloved" + meri "great, famous".
dragons: Many dragon species are still found in the wild throughout Europe. All appear to be are variations on the typical European dragon, with wings and fiery breath. Specific varieties mentioned so far are the Chinese Fireball, Common Welsh Green, Hebridean Black, Hungarian Horntail, Swedish Short-Snout, and Norwegian Ridgeback.
Although captive dragon breeding is now outlawed, dragons are still useful to wizards in many ways. Their heartstrings are used in wands, their dung is used as compost for magical plants, and Albus Dumbledore alone has invented twelve uses for their blood.
When facing a dragon, it is helpful to remember that its eyes are its weakest spot.
Dragon Breeding for Pleasure and Profit: One of Hagrid's collection of dragon books.
Dragon Keeper's Guide, A: Another of Hagrid's collection of dragon books.
dragon pox (OotP ch. 22): A contagious magical disease. I don't even want to think about how you get it.
Dragon Species of Great Britain and Ireland: Yet another of Hagrid's collection of dragon books.
Draught of Living Death: An extremely powerful sleeping potion.
Draught of Peace (OotP ch. 15): A calming potion.
Dreadful Denizens of the Deep: A book in the Hogwarts library.
Dream Oracle, The (OotP ch. 12): A book used in the Divination class in Harry's fifth year.
Dr. Filibuster's Fabulous Wet-Start, No-Heat Fireworks: Magical fireworks available at Gambol and Japes.
Dr. Ubbly's Oblivious Unction (OotP ch. 38): Applied to heal Harry's wounds from being attacked by a brain.
Etym: Ubbly might be another form of obley an archaic word for "offering, oblation, sacrifice".
Drooble's Best Blowing Gum: A wizard candy. Probably best to steer clear of it, because...
Etym: One of a number of variations on the French word for "trouble".
Drought Charm: A spell to lower or dry up small bodies of water.
Dudley Dursley: Etym: From an Old English placename, composed of the name Dudda + leeah "wood, clearing", and maybe a pun on him being a bit of a dud as a human being.
Dumbledore, Aberforth: Albus Dumbledore's brother, banned from using magic after being caught "practicing inappropriate charms on a goat".
Etym: See below.
Dumbledore, Albus Percival Wulfric Brian: A former headmaster of Hogwarts, old buddy of Grindelwald, a gourmet of candies, and discoverer of the 12 current uses for dragon's blood. An all-around nice guy, at least until he decides it's time for you to die.
Etym: Old word for a bumblebee, from dumble "stupid, dull, slow" + dor "insect that makes a loud humming noise".
In-depth article at HPLexicon
Dumbledore, Ariana (DH ch. 2): Albus and Aberforth's sister, who dutifully fulfilled the familial role of Relative That No One Talks About.
Etym: See above.
Dumbledore, Kendra (DH ch. 2): Albus, Aberforth, and Ariana's mother.
Etym: See above.
Dumbledore, Percival (DH ch. 2): Albus, Aberforth, and Ariana's father, who died in Azkaban due to the unfortunate circumstance of not living in an era when it was okay to beat up Muggles.
Etym: See above.
Dumbledore's Army (OotP ch. 18): Harry's secret extracurricular Defense Against the Dark Arts class. Members are Ron, Hermione, Hannah Abbott, Katie Bell, Susan Bones, Terry Boot, Lavender Brown, Cho Chang, Michael Corner, Colin Creevey, Dennis Creevey, Marietta Edgecombe, Justin Finch-Fletchley, Seamus Finnigan, Anthony Goldstein, Angelina Johnson, Lee Jordan, Neville Longbottom, Luna Lovegood, Ernie Macmillan, Padma Patil, Parvati Patil, Zacharias Smith, Alicia Spinnet, Dean Thomas, Fred and George Weasley, and Ginny Weasley.
Dundee: A city in northern Scotland.
Durmstrang Institute: Another school of magic, thought to be somewhere in northeastern Europe. Its curriculum is rumored to include dark magic.
Etym: Almost certainly invented from the German phrase sturm and drang.
Duro (DH ch. 32): An incantation to turn something to stone.
Etym: Latin, "I cause to become hard".
Dursley, Dudley: Harry's cousin, a fine young lad who knows how to get the best out of life, at least from people who are swayed by screaming tantrums.
Etym: Stated by Rowling to be from the place name, which for the record is from the Old English given name Deeorsige + leeah "wood, clearing".
Dursley, Marge: Harry's aunt-in-law, Vernon Dursley's sister, dog fancier, occasional babysitter, and dispenser of time-honored wisdom on the subject of breeding.
Etym: See above.
Dursley, Petunia Evans: Harry's aunt, Lily Potter's sister, who selflessly gave up several cubic feet of storage space to house her weird nephew.
Etym: See above.
Dursley, Vernon: A respectable businessman, the director of Grunnings, a doting father, and really not at all to blame for his nephew being a freak.
Etym: See above.