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. [1913 Webster] {Theta function} (Math.), one of a group of functions used in developing the properties of elliptic functions. [1913 Webster]