Ravenclaw House: One of the four houses of Hogwarts, extolling the virtues of wisdom and friendship. Its symbol is an eagle, and its colors are blue and silver. Attempting to combine this with the versions of its badge appearing in the movie and on merchandise gives Azure, an eagle displayed, wings inverted, argent, which really doesn't resemble either of those versions. The house ghost is the Grey Lady.
Etym: Invented. The raven is a symbol of wisdom in Norse mythology.
Ravenclaw, Rowena: One of the founders of Hogwarts.
Etym: See above.
Red Caps: Malicious spirits which lurk where there has been a battle, or other major bloodshed.
Reducio: An incantation which shrinks the target.
Etym: Latin, "I reduce". Imperative form: reducere "be reduced". Also, see below.
Reducto: The incantation for the Reductor Curse.
Etym: Akin to Latin reductio "reduction"; I suspect, though, that it's a typo for the above.
Reductor Curse: A spell that shrinks the target. Incantation (possibly): Reducto.
Registry of Proscribed Charmable Objects: A listing of Muggle artifacts that are illegal to enchant.
Relashio: An incantation that Harry attempted to fight a grindylow off with.
Etym: Not sure.
Remembrall: An item resembling a clear marble, which flashes red when the owner has forgotten something. It is unable to indicate what has been forgotten, though.
Remus J. Lupin:
Etym: In Roman legend, Romulus and Remus were orphaned or abandoned twins who were raised by a she-wolf and went on to found Rome (named after Romulus after he killed Remus).
Reparo: An incantation to fix minor damage.
Etym: Latin, "I repair". Imperative: repare "be repaired".
Restricted Section: A part of the Hogwarts library open only to faculty, advanced students of Defense Against the Dark Arts, and other students who can get a teacher's permission. Naturally, this is where all the really interesting and useful books are kept.
Revealer: An item that can make invisible ink visible.
Reverse Spell Effect: Another name for Priori Incantatem.
Rictusempra: A curse that makes the target laugh uncontrollably.
Etym: Rictus is Latin for "open mouth", and sempra is akin to various Romance languages' words for "always".
Riddle House: The house in Little Hangleton where Tom Riddle senior lived with his parents until all three were killed. The current owner of the house is a mysterious wealthy man who is believed to have bought it for tax reasons.
Riddle, Tom Marvolo: A star student and Head Boy about 50 years before Harry's time who opened the Chamber of Secrets and went on to a highly successful career as a master of dark wizardry under the name Voldemort.
Etym: OEW on an obscure meaning that I think is the one intended: "riddle
has been used for a thousand years to mean a dark saying ... and almost
as long to mean a coarse sieve, for arranging: separating chaff from
corn, ashes from cinders, etc."
Beastly equation for "Tom Riddle": a=12, b=18, c=24, etc.; a=43, b=46, etc.; a=105, b=102, etc.
and for "Thomas Riddle": a=26, b=29, etc.; a=85, b=82, etc.
Riddle, Tom, senior: Tom Riddle's father, a Muggle who abandoned his wife when he learned that she was a witch and went back to live with his own parents, until he and they were mysteriously murdered. The younger Riddle is still a bit put out about this. One of his bones was used to reconstitute Voldemort.
Riddikulus: An incantation used to counter a boggart.
Ripper (PoA ch. 2): One of Harry's Aunt Marge's favorite bulldogs.
Etym: As in Jack the...
Rise and Fall of the Dark Arts, The: A book in the Hogwarts library.
Rita Skeeter: Etym: A variation of Margaret.
Roberts (GoF ch. 7): One of the campsite managers for the Quidditch World Cup.
Etym: From Old English/German hrothi "fame" + berhta "bright".
rock cakes: A type of fruitcake, meant to be hard only on top but frequently ending up hard all the way through.
Roddy Pontner: Etym: Could be short for Roderick, from Old German hrothi "fame" + ricja "rule", or Rodney, from the village of Rodney Stoke.
Roger Davies: Etym: Derived from Old German Hrodgar, from hrothi "fame" and ger "spear". Name of various royalty.
Ron Weasley [Ronald]:
Etym: From a compound of the Old English words regen and weald, both of which mean "power, force, might".
Ronan: One of the centaurs living in the Forbidden Forest.
Etym: Gaelic for "seal".
Rookwood, Augustus (GoF ch. 30): A Death Eater who worked in the Department of Mysteries. Fate unknown.
Etym: A rook is a type of crow, as well as the chess piece. Rookwood is a romance by W. A. Ainsworth, recounting the exploits of highwayman Dick
rosewood: Name for several different ornamental timbers, never seen in large pieces because the heartwood of these trees decays early on. Its use it becoming even less common now due to dwindling supplies.
Rosier, Evan (GoF ch. 27): A Death Eater, killed before Voldemort's fall.
Etym: Poetic term for rose-tree or bush, of obscure origin.
Rosmerta, Madam: Proprietor of the Three Broomsticks.
Etym: Name of a Celtic goddess.
Rowena Ravenclaw: Etym: ECN: "This name seems to originate with Geoffrey of Monmouth, who gives it to the daughter of Hengist, with whom Vortigern fell in love." May be a compound of the Old English words hreod "fame" and wine "friend".
Rubeus Hagrid: Etym: From the Latin for "reddish".
Ryan (GoF ch. 8): The Keeper on the Irish national Quidditch team.
Etym: As a surname, derived ultimately from Rian or Riaghan, names of uncertain origin.