The Collaborative International Dictionary of English v.0.48:

Illegality \Il`le*gal"i*ty\, n.; pl. Illegalities. [Cf. F.
   ill['e]galit['e].]
   The quality or condition of being illegal; unlawfulness; as,
   the illegality of trespass or of false imprisonment; also, an
   illegal act.
   [1913 Webster]

WordNet (r) 3.0 (2006):

illegality
    n 1: unlawfulness by virtue of violating some legal statute
         [ant: legality]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "illegality":
   aberrance, aberrancy, abnormality, crime, criminality, delict,
   delinquency, deviance, deviancy, felony, improperness, impropriety,
   inappropriateness, incorrectness, indecorousness, indecorum,
   inequitableness, inequity, infraction, iniquitousness, iniquity,
   injustice, misdemeanor, offense, sinfulness, tort, undueness,
   unfitness, unfittingness, unjustness, unlawfulness, unmeetness,
   unrighteousness, unseemliness, unsuitability, violation,
   wickedness, wrong, wrongdoing, wrongfulness, wrongness