THETA

The″ta (?), n. [[L., fr. Gr. θη̑τα, the Greek letter θ, Θ.]] 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 θάνατοσ, death. Theta function (Math.), one of a group of functions used in developing the properties of elliptic functions.