The Collaborative International Dictionary of English v.0.48:

Legitimacy \Le*git"i*ma*cy\ (-[i^]*m[.a]*s[y^]), n. [See
   Legitimate, a.]
   The state, or quality, of being legitimate, or in conformity
   with law; hence, the condition of having been lawfully
   begotten, or born in wedlock.
   [1913 Webster]

         The doctrine of Divine Right, which has now come back
         to us, like a thief from transportation, under the
         alias of Legitimacy.                     --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

legitimacy
    n 1: lawfulness by virtue of being authorized or in accordance
         with law [ant: illegitimacy]
    2: undisputed credibility [syn: authenticity, genuineness,
       legitimacy]