\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.