The Collaborative International Dictionary of English v.0.48:

Treason \Trea"son\, n. [OE. tresun, treisun, traisoun, OF.
   tra["i]son, F. trahison, L. traditio a giving up, a
   delivering up, fr. tradere to give up, betray. See Traitor,
   and cf. Tradition.]
   1. The offense of attempting to overthrow the government of
      the state to which the offender owes allegiance, or of
      betraying the state into the hands of a foreign power;
      disloyalty; treachery.
      [1913 Webster]

            The treason of the murthering in the bed. --Chaucer.
      [1913 Webster]

   Note: In monarchies, the killing of the sovereign, or an
         attempt to take his life, is treason. In England, to
         imagine or compass the death of the king, or of the
         queen consort, or of the heir apparent to the crown, is
         high treason, as are many other offenses created by
         statute. In the United States, treason is confined to
         the actual levying of war against the United States, or
         to an adhering to their enemies, giving them aid and
         comfort.
         [1913 Webster]

   2. Loosely, the betrayal of any trust or confidence;
      treachery; perfidy.
      [1913 Webster]

            If he be false, she shall his treason see.
                                                  --Chaucer.
      [1913 Webster]

   Petit treason. See under Petit.
      [1913 Webster]

WordNet (r) 3.0 (2006):

treason
    n 1: a crime that undermines the offender's government [syn:
         treason, high treason, lese majesty]
    2: disloyalty by virtue of subversive behavior [syn: treason,
       subversiveness, traitorousness]
    3: an act of deliberate betrayal [syn: treachery, betrayal,
       treason, perfidy]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "treason":
   Golconda, Machiavellianism, apostasy, backsliding, betrayal, bolt,
   breach of trust, breakaway, collaboration, crossing-over, deceit,
   deceitfulness, defection, degeneration, desertion, disloyalty,
   duplicity, eldorado, faithlessness, fifth-column activity,
   fraternization, going over, gold mine, high treason, lese majesty,
   mine, misprision, misprision of treason, perfidiousness, perfidy,
   petty treason, quislingism, ratting, recidivation, recidivism,
   recreancy, renunciation, secession, sedition, seditiousness,
   treacherousness, treachery, treasure, treasure trove,
   treasure-house, treasury, turning traitor



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

TREASON, crim. law. This word imports a betraying, treachery, or breach of
allegiance. 4 Bl. Com. 75.
     2. The constitution of the United States, art. 3, s. 3, defines treason
against the United States to consist only in levying war (q.v.) against
them, or in adhering to their enemies, giving them aid or comfort. This
offence is punished with death. Act of April 30th, 1790, 1 Story's Laws U.
S. 83. By the same article of the constitution, no person shall be convicted
of treason, unless on the testimony of two witnesses to the same overt act,
or on confession in open court. Vide, generally, 3 Story on the Const. ch.
39, p. 667; Serg. on the Const. ch. 30; United States v. Fries, Pamph.; 1
Tucker's Blackst. Comm. Appen. 275, 276; 3 Wils. Law Lect. 96 to 99; Foster,
Disc. I; Burr's Trial; 4 Cranch, R. 126, 469 to 508; 2 Dall. R. 246; 355; 1
Dall. Rep. 35; 3 Wash. C. C. Rep. 234; 1 John. Rep. 553 11 Johns. R. 549;
Com. Dig. Justices, K; 1 East, P. C. 37 to 158; 2 Chit. Crim. Law, 60 to
102; Arch. Cr. Pl. 378 to 387.