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".
Canary Creams: A pilot product for Weasley's Wizard Wheezes which turns the eater into a giant canary.
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.
Cauldwell, Owen (GoF ch. 12): A Hufflepuff, three years behind Harry.
Etym: The name of several places, variously spelled but all from Old English c(e)ald "cold" + well(a) "spring". As a common noun, a weir that diverts water into a mill-lead.
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.
chamberpot room: Dumbledore claims to have stumbled into a hidden room filled with solid gold chamberpots one night when he was heading for the bathroom.
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.
Chinese Fireball: A variety of dragon.
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.
chocolate gateau: A type of chocolate cake. Here's an example, although I'm not sure how representative it is.
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.
cobbing: A penalizable infraction in Quidditch-- excessive use of elbows toward opponents.
Etym: The word is first noted as a term for a nautical punishment in the late 18th century, taking on the more general meaning "to strike" in the mid-19th.
cockatrice: In antiquity, originally another name for a basilisk. Later on, a creature with the head, wings, and feet of a rooster, a serpentine body, and a barbed tail.
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 Magical Ailments and Afflictions: May or may not be an actual book available in the wizarding world.
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.
Conjunctivitis Curse: Conjunctivitis is inflammation of the conjunctiva-- the inner membrane of the eyelid.
conk: British slang for "nose".
Connolly (GoF ch. 8): A Beater on the Irish national Quidditch team.
Etym: Anglicized form of Ó Conghalaigh, from a name meaning "valiant".
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 pasty: A pasty is a small pastry filled with meat, potatoes, and vegetables. Pronounced to rhyme with "nasty".
Cornish pixies: Small electric-blue creatures; not terribly dangerous, although they seem to have a talent for vandalism.
Crabbe senior (GoF ch. 33): A Death Eater still at large.
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.
Cribbages Wizarding Crackers: A magical version of Christmas crackers-- traditional British party favors consisting of a wrapped tube containing a surprise.
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.
Creevey, Dennis: A Gryffindor, 3 years behind Harry.
Etym: See above.
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.
crisps: Bits of pastry made by deep-frying batter.
Croaker (GoF ch. 7): An Unspeakable seen at the Quidditch World Cup.
Etym: As a common noun, one who talks dismally or despondingly, or who forebodes or prophesies evil. Makes you worry what they're up to in the Department of Mysteries.
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".
Crouch, Bartemius, junior: A convicted Death Eater who was snuck out of Azkaban by his father and went on to repay him by mind-controlling him, usurping his identity, and eventually killing him.
Etym: In addition to the usual meanings, an obsolete form of cross; name for someone who lived near a cross.
Crouch, Bartemius, senior: The former head of the Department of Magical Law Enforcement, later Percy Weasley's boss in the Department of International Magical Cooperation, still later killed by his own son.
Etym: See Crouch junior.
Cruciatus Curse: One of the Unforgivable Curses, it causes unbearable pain to the target. Prolonged exposure can apparently result in memory loss, as for the Longbottoms. Incantation: Crucio.
Crucio: The incantation for the Cruciatus Curse.
Etym: Latin, literally "I crucify". The same word at the root of exruciating. Imperative: crucire "be crucified".
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.
Cuthbert Mockridge: Etym: St. Cuthbert (d. 687) was bishop of Lindisfarne and is one of the most popular saints in northern England. Also a term for a conscientious objector in World War I.