·noun One who believes in the doctrine of the Trinity.
II. Trinitarian ·adj Of or pertaining to the Trinity, the doctrine of the Trinity, or believers in that doctrine.
III. Trinitarian ·noun One of a monastic order founded in Rome in 1198 by St. John of Matha, and an old French hermit, Felix of Valois, for the purpose of redeeming Christian captives from the Mohammedans.