Lewis & Short

thēta, indecl. n., = θῆτα, the Greek letter θ; as the initial letter of the word θάνατος (death), written by the Greeks upon their voting-tablets in sign of condemnation, Mart. 7, 37, 2; Aus. Epigr. 128 (cf. Pers. 4, 13).
Upon Latin epitaphs, = OBIIT, MORTVVS EST, Inscr. Orell. 4472 sq.; cf. Marin. Fratr. Arv. p. 610.
Appended to a passage as a critical mark of censure, Sid. Carm. 9, 335.