Source : Webster's Revised Unabridged Dictionary (1913)

Traitor \Trai"tor\, n. [OE. traitour, OF. tra["i]tor,
   tra["i]teur, F. tre[^i]tre, L. traditor, fr. tradere,
   traditum, to deliver, to give up or surrender treacherously,
   to betray; trans across, over + dare to give. See {Date}
   time, and cf. {Betray},{Tradition}, {Traditor}, {Treason}.]
   1. One who violates his allegiance and betrays his country;
      one guilty of treason; one who, in breach of trust,
      delivers his country to an enemy, or yields up any fort or
      place intrusted to his defense, or surrenders an army or
      body of troops to the enemy, unless when vanquished; also,
      one who takes arms and levies war against his country; or
      one who aids an enemy in conquering his country. See
      {Treason}.

            O passing traitor, perjured and unjust! --Shak.

   2. Hence, one who betrays any confidence or trust; a
      betrayer. ``This false traitor death.'' --Chaucer.

Traitor \Trai"tor\, a.
   Traitorous. [R.] --Spenser. Pope.

Traitor \Trai"tor\, v. t.
   To act the traitor toward; to betray; to deceive. [Obs.] ``
   But time, it traitors me.'' --Lithgow.

Source : WordNet®

traitor
     n 1: someone who betrays his country by committing treason [syn:
          {treasonist}]
     2: a person who says one thing and does another [syn: {double-crosser},
         {double-dealer}, {two-timer}, {betrayer}]