Source : Webster's Revised Unabridged Dictionary (1913)

Unitarian \U`ni*ta"ri*an\, n. [Cf. F. unitaire, unitairien, NL.
   unitarius. See {Unity}.]
   1. (Theol.) One who denies the doctrine of the Trinity,
      believing that God exists only in one person; a
      unipersonalist; also, one of a denomination of Christians
      holding this belief.

   2. One who rejects the principle of dualism.

   3. A monotheist. [R.] --Fleming.

Unitarian \U`ni*ta"ri*an\, a.
   Of or pertaining to Unitarians, or their doctrines.

Source : WordNet®

Unitarian
     adj : of or relating to or characterizing Unitarianism

Unitarian
     n : adherent of Unitarianism