[ noun ] the 8th letter of the Greek alphabet <noun.communication>
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.