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.
Cauldron Cakes: A type of wizard candy, probably something akin to a cupcake.
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.
Etym: From ceorl, Old English for "a man".
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.
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.
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.
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.
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 on Experimental Charms: Part of the Ministry of Magic.
Common Welsh Green: A type of dragon native to the British Isles.
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.
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.