Source : Webster's Revised Unabridged Dictionary (1913)

Illegitimacy \Il`le*git"i*ma*cy\, n.
   The state of being illegitimate. --Blackstone.

Source : WordNet®

illegitimacy
     n 1: the status of being born to parents who were not married
          [syn: {bastardy}, {bar sinister}]
     2: unlawfulness by virtue of not being authorized by or in
        accordance with law [ant: {legitimacy}]