Dictionary.net

• tri-tag'on-ist, n. the third actor in the Greek drama. [Gr. tritos, third, agonist[=e]s, an actor.]

1908 - Chambers's Twentieth Century Dictionary of the English Language
By Thomas Davidson

• Third actor in Greek play (cf. DEUTERAGONIST). [Greek]

1919 - The concise Oxford dictionary of current English
By Sir Augustus Henry