Tarantallegra: An incantation that makes the target dance uncontrollably.
Etym: From Italian tarantella, a type of dance, and allegra "fast" (also a musical term).
Ted [Theodore] (PS ch. 1): Name of a newsreader the Dursleys watch.
Etym: Greek, "god's gift".
Terence Higgs: Etym: From Latin Terentius, the name of a Roman gens of unknown etymology.
Etym: See above.
Thomas, Dean (PS ch. 7): A Gryffindor, the same year as Harry. From a Muggle family, he is a big fan of the West Ham soccer team.
Etym: From Aramaic meaning "twin". We can also consider Thomas the Rhymer, a 13th century Scottish poet and prophet who in popular lore is connected with Merlin.
Three Broomsticks: The pub in Hogsmeade.
Tibbles (PS ch. 2): One of Mrs. Figg's cats.
Timms, Agatha (GoF ch. 7): Etym: Etymology unknown; possibly cognate to the Germanic name Timmo, of unknown meaning.
Tom (PoA ch. 3): The proprietor of The Leaky Cauldron.
Etym: From an Aramaic word meaning "twin". One of the twelve apostles, whose real name may have been Judah; the nickname would have been used to distinguish him from Judah the brother of James (St. Jude), and Judah of Kerioth (Judas Iscariot).
Ton-Tongue Toffee: A joke candy created by Fred and George Weasley which enlarges the eater's tongue beyond all belief.
Toothflossing Stringmints: A type of candy available at Honeydukes.
trading cards: See Famous Witches and Wizards.
Transfiguration: One of the basic subjects at Hogwarts, it concerns the changing of things from one form to another. The Transfiguration teacher is Professor McGonagall.
Transfiguration Today: A wizard publication.
Transmogrifian Torture: A deadly spell that Lockhart claimed to know about.
Travels with Trolls: One of Gilderoy Lockhart's books.
Travers: A Death Eater, imprisoned in Azkaban.
Etym: Name for someone who lived by a bridge or ford, or a gatherer of tolls, from Middle English/Old French travers "passage, crossing". Or, an Anglicized form of Gaelic Ó Treabhair, from a byname meaning "industrious, prudent".
treacle tart: A syrupy pastry usually served with custard.
Trelawney, Sibyll: The Divination instructor and constant predictor of woe.
Etym: Placename, from Cornish tre "homestead, settlement" + an element of unknown meaning.
Trevor (PS ch. 6): Neville Longbottom's toad.
Etym: From Welsh tre(f) "homestead, settlement" + mawr "large", or Gaelic Ó Treabhair, from a byname meaning "industrious, prudent".
trifle: A confection made of layers of sherry-soaked sponge cake, jelly, and custard, usually topped with whipped cream and fruit.
Trimble, Quentin (PS ch. 5): Author of The Dark Forces: A Guide to Self-Protection.
Etym: From an Old English personal name, formed from trum "strong, firm" + beald "bold, brave". On the other hand, also an obsolete form of tremble.
Triwizard Cup: The trophy of the Triwizard Tournament.
Triwizard Tournament: A competition between champions selected from Beauxbatons, Durmstrang, and Hogwarts chosen by the Goblet of Fire. Originally established 700 years ago and meant to be held every five years, but discontinued in recent centuries owing to a high fatality rate. It was resurrected in Harry's fourth year.
trolls: Large, stupid humanoid creatures that like to fight with clubs.
Troy (GoF ch. 8): A Chaser on the Irish national Quidditch team.
Etym: An Anglicized form of Ó Troighthigh, from a byname meaning "foot soldier".
True Seer: One with the gift of prophecy.
tubeworms: This term covers a whole range of polychaete worms.
Tufty (PS ch. 2): One of Mrs. Figg's cats.
Turpin, Lisa (PS ch. 7): A Ravenclaw, same year as Harry.
Etym: A number of possibilities here, but my favorite is Dick Turpin (1706-1739), an English highwayman, the subject of Rookwood.
Twitchy Ears: A curse that makes the target's ears wiggle.