Theta /(?)/

The·ta

Theta

n.
  1. 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.

Phrases & Compounds
Theta function
one of a group of functions used in developing the properties of elliptic functions.