n. [ L., fr. Gr. qh^ta, 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 qa`natos, death. [ 1913 Webster ]

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