The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

theta
    n 1: the 8th letter of the Greek alphabet