Rabastan Lestrange: Etym: Probable corruption of Rastaban, a star whose name means "head of the snake".
Rackharrow, Urquhart (OotP ch. 22): The inventor of the Entrail-Expelling Curse. Wonder if he survived the experience.
Etym: Rack as in a racking cough, harrow as in "torment" or its archaic sense of "plunder".
Ragnok (OotP ch. 5): A goblin leader in negotiations with the Order of the Phoenix.
Etym: Diminutive form of Ragnarok??
Ragnuk the First (DH ch. 25): According to the goblins, the original owner of Godric Gryffindor's sword.
Etym: Maybe supposed to be the same as the previous.
Railview Hotel: A place where the Dursleys attempted to hide themselves and Harry from the Hogwarts owls.
Rapier (DH ch. 22): Fred Weasley's code name on Potterwatch.
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.
Ravenclaw, Helena: Rowena's daughter and slightly too impatient heir.
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.
Refilling Charm (HBP ch. 22): A spell to regenerate whatever a bottle previously contained.
Registry of Proscribed Charmable Objects: A listing of Muggle artifacts that are illegal to enchant.
Reg Cattermole [Reginald] (DH ch. 12):
Etym: Germanic, ragin "advice, decision" + wald "ruler".
Regulus Black: Etym: A star of great astrological importance. Its name is Latin for "little king".
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).
Renervate (HBP ch. 26): A spell to awaken a person. This may be Enervate under another new spelling.
Rennervate (HBP ch. 26): Renervate as spelled by Scholastic.
Reparo: An incantation to fix minor damage.
Etym: Latin, "I repair". Imperative: repare "be repaired".
Repello Muggletum (DH ch. 14): The incantation for a Muggle Repelling Charm.
Etym: Pseudo-Latin for "I repel Muggles".
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.
Resurrection Stone (DH ch. 21): Created by Cadmus Peverell, turned into a Horcrux by Voldemort, de-Horcruxed by Albus Dumbledore, but still apparently capable of establishing contact with the dead. Last seen in the forest not too far from Hogwarts, if you happen to be in the area.
Reusable Hangman (HBP ch. 6): Another product of Weasley's Wizard Wheezes.
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.
Ritchie Coote: Etym: Diminutive of Richard from ric "ruler" + heard "hard".
River (DH ch. 22): Lee Jordan's Potterwatch code name.
Robards, Gawain: Scrimgeour's successor as the head Auror.
Etym: See Roberts.
Roberts (GoF ch. 7): One of the campsite managers for the Quidditch World Cup.
Etym: From Old English/German hrothi "fame" + berhta "bright".
Robins, Demelza (HBP ch. 11): The newest Chaser on the Gryffindor Quidditch team.
Etym: Diminutive of Robert (see Roberts).
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.
Rodent (DH ch. 22): Fred Weasley's original assigned code name for Potterwatch.
Rodolphus Lestrange: Etym: From Old German hrothi "fame" + vulf "wolf".
Roger Davies: Etym: Derived from Old German Hrodgar, from hrothi "fame" and ger "spear". Name of various royalty.
Romilda Vane: Etym: An Italian name ultimately derived from Old German hroom "fame" + hild "battle".
Romulus (DH ch. 22): Remus Lupin's code name on Potterwatch.
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: A Death Eater who worked in the Department of Mysteries until being caught and imprisoned in Azkaban. Ran loose for a brief time, until being rounded up at the Ministry of Magic and sent back to Azkaban again.
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 Turpin and his mare Black Bess.
Room of Requirement (OotP ch. 18): A room on the seventh floor of Hogwarts that appears when necessary equipped with whatever the seeker desperately needs. Presumably there are limitations, since it isn't also known as the Room of Deus Ex Machina.
Rosalind Antigone Bungs: Etym: From Old German (h)ros "horse" + lindi "serpent"; the name of a character in As You Like It.
Rose Weasley: Etym: See below.
Rose Zeller: Etym: The name actually comes from (h)ros "horse", but over time has become associated with the flower.
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".
Rowle, Thorfinn (DH ch. 9): One of the the hardier Death Eaters.
Etym: From Middle English hrood "renown" + wulf "wolf".
Royal (DH ch. 22): Kingsley Shacklebolt's code nam on Potterwatch.
Rubeus Hagrid: Etym: From the Latin for "reddish".
Rufus Scrimgeour: Etym: Latin, "red-haired".
Runcorn, Albert (DH ch. 12): A big mover and shaker at the Ministry of Magic, and I mean big.
Etym: The name of a New Town built in Cheshire to hold overflow population for Liverpool and Manchester. For English romanticists, New Towns have sinister implications.
Rupert Brookstanton: Etym: From an Old German form of Robert (see Roberts).
Ryan, Barry (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.