The Collaborative International Dictionary of English v.0.48:

Transgression \Trans*gres"sion\, n. [L. transgressio a going
   across, going over, transgression of the law, from
   transgredi, transgressus, to step across, go over; trans
   over, across + gradi to step, walk: cf. F. transgression. See
   Grade.]
   The act of transgressing, or of passing over or beyond any
   law, civil or moral; the violation of a law or known
   principle of rectitude; breach of command; fault; offense;
   crime; sin.
   [1913 Webster]

         Forgive thy people . . . all their transgressions
         wherein they have transgressed against thee. --I Kings
                                                  viii. 50.
   [1913 Webster]

         What rests, but that the mortal sentence pass
         On his transgression, death denounced that day ?
                                                  --Milton.
   [1913 Webster]

         The transgression is in the stealer.     --Shak.
   [1913 Webster]

   Syn: Fault; offense; crime; infringement; misdemeanor;
        misdeed; affront; sin.
        [1913 Webster]

WordNet (r) 3.0 (2006):

transgression
    n 1: the act of transgressing; the violation of a law or a duty
         or moral principle; "the boy was punished for the
         transgressions of his father" [syn: transgression,
         evildoing]
    2: the spreading of the sea over land as evidenced by the
       deposition of marine strata over terrestrial strata
    3: the action of going beyond or overstepping some boundary or
       limit

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "transgression":
   atrocity, bad faith, breach, breach of contract, breach of faith,
   breach of law, breach of privilege, breach of promise,
   breach of trust, breaking, civil disobedience, contravention,
   crime, crime against humanity, deadly sin, delinquency,
   dereliction, disobedience, encroachment, enormity, erring, error,
   evil, failure, fall from grace, fault, felony, frowardness,
   genocide, guilty act, heavy sin, impropriety, incursion,
   indiscipline, indiscretion, indocility, inexpiable sin, infraction,
   infringement, iniquity, injury, injustice, inroad, insubordination,
   intractability, intrusion, lapse, lawbreaking, lawlessness,
   malefaction, malfeasance, malum, minor wrong, misbehavior, misdeed,
   misdemeanor, misfeasance, mortal sin, naughtiness, noncompliance,
   nonconformity, noncooperation, nonfeasance, nonobedience, offense,
   omission, outrage, overstepping, passive resistance, peccadillo,
   peccancy, recusancy, sin, sin of commission, sin of omission,
   sinful act, slip, tort, trespass, trespassing, trip,
   uncooperativeness, unduteousness, undutifulness, unsubmissiveness,
   unutterable sin, usurpation, venial sin, violation,
   violation of law, waywardness, willful disobedience, wrong,
   wrongdoing



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

TRANSGRESSION. The violation of a law.