The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            O passing traitor, perjured and unjust! --Shak.
      [1913 Webster]

   2. Hence, one who betrays any confidence or trust; a
      betrayer. "This false traitor death." --Chaucer.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Traitor \Trai"tor\, a.
   Traitorous. [R.] --Spenser. Pope.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Traitor \Trai"tor\, v. t.
   To act the traitor toward; to betray; to deceive. [Obs.] "
   But time, it traitors me." --Lithgow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "traitor":
   Benedict Arnold, Brutus, Judas, Judas Iscariot, Quisling, agitator,
   apostate, archtraitor, backslider, betrayer, bolter, brawler,
   cockatrice, collaborationist, collaborator, conniver, conspirator,
   conspirer, convert, convict, criminal, crook, deceiver, defector,
   deserter, desperado, desperate criminal, double agent,
   double-crosser, double-dealer, extremist, felon, fifth columnist,
   frondeur, fugitive, gallows bird, gangster, gaolbird, informer,
   insubordinate, insurgent, insurrectionary, insurrectionist,
   insurrecto, intrigant, intriguer, jailbird, lawbreaker, machinator,
   malcontent, maverick, mobster, mugwump, mutineer, nonconformist,
   outlaw, plotter, proselyte, public enemy, quisling, racketeer, rat,
   rebel, recidivist, recreant, renegade, renegado, renegate,
   reversionist, revolter, revolutionary, revolutionist, rioter,
   runagate, schemer, schismatic, scofflaw, scoundrel, seceder,
   secessionist, separatist, serpent, snake, strikebreaker,
   subversive, swindler, tergiversant, tergiversator, thief, thug,
   timeserver, treasonist, trimmer, turnabout, turncoat, turntail,
   two-timer



Bouvier's Law Dictionary, Revised 6th Ed (1856):

TRAITOR, crimes. One guilty of treason.
     2. The punishment of a traitor is death.