Cadogan, Sir: A knight whose portrait was temporarily moved to guard the Gryffindor dormitory when the Fat Lady was frightened off.
Etym: From the Old Welsh name Cadoc, which is possibly related to a word meaning "battle".
Caput Draconis: The first password to the Gryffindor tower when Harry arrives at Hogwarts.
Etym: Latin for "dragon's head".
Care of Magical Creatures: An elective class at Hogwarts, taught by Professor Kettleburn until Harry's third year, when Hagrid took over. Harry, Ron, and Hermione are all enrolled in it.
Cassandra Vablatsky: Etym: In Greek legend, Cassandra was given the gift of prophecy by the god Apollo, who loved her, and then cursed by him when she rejected him. The curse was that no one would ever believe her predictions.
Cauldron Cakes: A type of wizard candy, probably something akin to a cupcake.
Cedric Diggory: Etym: ECN: "This now not uncommon name seems to have been invented by Sir Walter Scott for one of the characters in Ivanhoe, `Cedric the Saxon'. It was probably a mistake of Scott's for Cerdic, the name of the traditional founder of the West Saxon kingdom." Which may in turn be from the Welsh name Caradawg, meaning "amiable".
Celestina Warbeck: Etym: This was the term for a late-18th-century keyboard instrument developed from the armonica. The word is from Latin caelestis "heavenly".
centaur: A mythical creature with the body of a horse, and the top half of a human attached where the neck would be. Rowling's centaurs are largely peaceful but wish to keep to themselves. A group of them lives in the Forbidden Forest.
Chamber of Secrets: An enormous magical cavern, rumored to have been constructed by Salazar Slytherin, concealed beneath Hogwarts for nearly eight centuries, even when a modern girls' bathroom was built over the entrance, until Tom Riddle figured out how to open it and pin the blame on Hagrid.
Chameleon Ghoul: Mentioned in passing, presumably a type of ghoul with some natural ability to disguise itself.
Chang, Cho: A Ravenclaw, a year ahead of Harry, and Seeker for the Ravenclaw Quidditch team.
Etym: No reliable info.
Charing Cross Road: A major road in London. The Leaky Cauldron is apparently located along or near it.
Etym: From ceorl, Old English for "a man".
Charm to Cure Reluctant Reversers, A: Page 12 of the Handbook of Do-It-Yourself Broomcare.
Charms: One of the basic subjects that all students at Hogwarts have to study, taught by Professor Flitwick. A general term for all sorts of minor spells of instantaneous effect.
Charm Your Own Cheese: One of Molly Weasley's cookbooks.
Chaser: One of the three players on a Quidditch team who pass the Quaffle between them and attempt to throw it through one of the goal hoops. A goal is worth 10 points.
Cheering Charm: A charm to elevate someone's mood.
chipolata: A type of sausage.
Cho Chang: Etym: No reliable info on the meaning of the name. It was the family name of a couple of Korean artists of the Yi dynasty.
Chocoballs: A candy available at Honeydukes.
Chocolate Frogs: A wizard candy, undistinguished except for the line of Famous Witches and Wizards trading cards that come with them.
Christmas pudding: A rich fruit pudding, typically splashed with brandy and set alight just before serving. Hiding a silver coin in it is also traditional.
Chudley Cannons: A professional Quidditch team. Chudley itself appears to be fictional, unless it's an alternate spelling of Chudleigh.
Circe: An enchantress who figures in the Odyssey. She transformed Odysseus's crew into pigs. Featured on a Famous Witches and Wizards trading card.
Cleansweep Five: A type of flying broom.
Cleansweep Seven: A type of flying broom.
Clearwater, Penelope (CoS ch. 14): Percy Weasley's girlfriend, a Ravenclaw prefect.
Etym: Pretty much what it looks like, and like her first name, a symbol of purity. Just the girl for Percy...
Cliodna: A druid featured on a Famous Witches and Wizards trading card.
Cockroach Cluster: A candy available at Honeydukes.
Etym: Likely from a product of the same name mentioned in the Monty Python "Crunchy Frog" sketch (series 1, episode 6).
Cokeworth: Location of the Railview Hotel.
Colin Creevey: Etym: Anglicization of Gaelic Cailean, which may be from coileán, meaning "young dog, youth" and by extension "cadet".
Comet Two Sixty: A brand of flying broom.
Committee for the Disposal of Dangerous Creatures: Part of the Ministry of Magic. "Disposal" usually consists of killing the animal.
Committee on Experimental Charms: Part of the Ministry of Magic.
Common Welsh Green: A type of dragon native to the British Isles.
Confundus Charm: A spell that can be used to temporarily make a person believe something they would otherwise disbelieve.
Confusing Concoction: A type of potion Harry had to make for his final exam in his third year.
conk: British slang for "nose".
Cornelius Fudge: Etym: St. Cornelius was pope from 251 to 253, and is noted for taking a liberal attitude toward Christians who had renounced their faith under duress. He was succeeded by St. Lucius.
Cornish pixies: Small electric-blue creatures; not terribly dangerous, although they seem to have a talent for vandalism.
Crabbe, Vincent: One of Draco Malfoy's cronies.
Etym: From a nickname for someone with an odd gait, like a crab, or a cantankerous person, as a shortening of crabapple.
Creevey, Colin: A Gryffindor, a year behind Harry; Harry's first papparazzo.
Etym: Anglicized form of Gaelic Ó Craoibhe, from a name meaning "curly(-headed)" or "prolific". Thomas Creevey (1768-1838) was a politician and placeman, remembered because some of his journals and correspondence were published in 1903 and 1905.
crinolines: Clothing made with crinoline, a stiff fabric made of horsehair that was used primarily in the 19th century. At first it was used in hats and shoes, and later to make dresses, petticoats, and other things as a substitute for stiffened muslin.
Crockford, Doris (PS ch. 5): A particularly effusive fan of Harry's that he met in The Leaky Cauldron.
Etym: From the placename Crockford Bridge; further etymology is uncertain and probably not relevant. Also the name of a London gambling club, and the colloquial designation of a reference work produced by the Anglican Church.
Crookshanks: Hermione's unusually intelligent cat.
Etym: Means "crooked legs".
curses: Spells that injure or impede the target. Some require only a wand and a quick incantation, but stronger ones require the caster to maintain line-of-sight and keep up the incantation for the full time of effect. The terms jinx and hex appear to be equivalent, and are used in curse names for alliterative effect. : A book seen in Diagon Alley.
Curse of the Bogies: Something Professor Quirrell mentioned in class. Bogie has varied meanings, a lot of them overlapping with boggart.