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.
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).
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.
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.
trolls: Large, stupid humanoid creatures that like to fight with clubs.
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.