D defs.my
Entry 1 sense Webster, 1913

Theta

/thāt'-ə/ · The·ta · IPA /ˈθeɪtə/
01 n. A letter of the Greek alphabet corresponding to th in English; -- sometimes called the unlucky letter, from being used by the judges on the…
  1. 1.
    A letter of the Greek alphabet corresponding to th in English; -- sometimes called the unlucky letter, from being used by the judges on their ballots in passing condemnation on a prisoner, it being the first letter of the Greek qa`natos, death.
Phrases & compounds
Theta function — one of a group of functions used in developing the properties of elliptic functions.