Quality Quidditch Supplies: A store in Diagon Alley.
Quentin Trimble: Etym: From Latin for "fifth"; also the name for a type of French linen around the end of the 17th century.
Quibbler, The (OotP ch. 7): A wizard tabloid specializing in news of dubious provenance, kind of a magical equivalent to The Sun.
Quick-Quotes Quill: An indispensable part of Rita Skeeter's arsenal which eliminates all the tiresome rote work of actually writing turgid prose and lets her get straight to the libel and slander.
Quidditch: The most popular sport in the wizarding world, combining, on the one hand, the fast-moving team action of soccer or basketball with, on the other hand, the injury rate of rugby and the vehicular technology arms race of Formula One.
Briefly, a game takes place on brooms between two teams, each consisting of three Chasers, two Beaters, a Keeper, and a Seeker, using three types of ball: a Quaffle, two Bludgers, and the Golden Snitch. For a full treatment of the subject, see Quidditch Through the Ages or Quidditch.com.
Quidditch Teams of Britain and Ireland: A book that Hermione gave Harry for Christmas.
Quidditch Through the Ages: The definitive work on the history of Quidditch, available in a Muggle edition.
Quidditch World Cup: An international Quidditch competition held every four years.
Quietus: An incantation that cancels out the effect of Sonorus.
Etym: Latin, "quiet" (as a noun).
Quigley (GoF ch. 8): A Beater on the Irish national Quidditch team.
Etym: Anglicized form of Ó Coighligh, from a byname meaning "untidy person".
Quintessence: A Quest (HBP ch. 15): Assigned reading for Charms; not sure why, as the quintessence is usually associated with alchemy.
Quirke, Orla (GoF ch. 12): A Ravenclaw, 3 years behind Harry.
Etym: Anglicized form of Ó Cuirc, from a personal name meaning "heart" or, possibly, "tuft of hair". Sometimes translated as if from coirce, "oats".
Quirrell: The last name of the teacher of Defense Against the Dark Arts in Harry's first year; revealed as a servant of Voldemort and killed in a confrontation with Harry.
Etym: Nickname for a troublemaker, from Middle English/Old French querel, meaning "complaint, accusation".