From The Collaborative International Dictionary of English v.0.48 :
Illegitimacy \Il`le*git"i*ma*cy\, n.
The state of being illegitimate. --Blackstone.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
illegitimacy
n 1: the status of being born to parents who were not married
[syn: bastardy, illegitimacy, bar sinister]
2: unlawfulness by virtue of not being authorized by or in
accordance with law [ant: legitimacy]