Source : Webster's Revised Unabridged Dictionary (1913)

Trinitarian \Trin`i*ta"ri*an\, a. [Cf. F. trinitaire. See
   {Trinity}.]
   Of or pertaining to the Trinity, the doctrine of the Trinity,
   or believers in that doctrine.

Trinitarian \Trin`i*ta"ri*an\, n.
   1. One who believes in the doctrine of the Trinity.

   2. (Eccl. Hist.) 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.

Redemptionist \Re*demp"tion*ist\, n. (R.C.Ch.)
   A monk of an order founded in 1197; -- so called because the
   order was especially devoted to the redemption of Christians
   held in captivity by the Mohammedans. Called also
   {Trinitarian}.

Source : WordNet®

Trinitarian
     n : adherent of Trinitarianism