Theta \The"ta\, n. [L., fr. Gr. qh^ta, the Greek letter [theta], [Theta].]

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.

Theta function (Math.), one of a group of functions used in developing the properties of elliptic functions.

Source: Webster's Revised Unabridged Dictionary (1913)