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]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "legitimacy":
   absolute power, absolute realism, absolutism, actionability,
   admissibility, admissibleness, allowableness, applicability,
   artlessness, authenticity, authority, bona fideness, competence,
   competency, constituted authority, constitutional validity,
   constitutionalism, constitutionality, defensibility,
   delegated authority, divine right, due process, excusability,
   explainability, explicability, faculty, forgivableness,
   genuineness, honesty, inartificiality, indirect authority,
   inherent authority, jurisdiction, jus divinum, justice,
   justiciability, justifiability, justifiableness, lawful authority,
   lawfulness, legal authority, legal form, legal process, legalism,
   legality, legitimateness, licitness, lifelikeness, literalism,
   literality, literalness, naturalism, naturalness, pardonableness,
   permissibility, permissibleness, photographic realism, power,
   prerogative, realism, realness, reasonability, reasonableness,
   regality, remissibility, right, rightful authority, rightfulness,
   royal prerogative, sanctionableness, scope, sincerity, the say,
   the say-so, true-to-lifeness, truth to nature, unadulteration,
   unaffectedness, unfictitiousness, unspeciousness, unspuriousness,
   unsyntheticness, validity, veniality, verisimilitude,
   vested authority, vicarious authority, vindicability,
   warrantableness



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

LEGITIMACY. The state of being born in wedlock; that is, in a lawful manner.
     2. Marriage is considered by all civilized nations as the only source
of legitimacy; the qualities of husband and wife must be possessed by the
parents in order to make the offspring legitimate; and furthermore the
marriage must be lawful, for if it is void ab initio, the children who may
be the offspring of such marriage are not legitimate. 1 Phil. Ev. Index,
h.t.; Civ. Code L. art. 203 to 216.
     3. In Virginia, it is provided by statute of 1787, "that the issue of
marriages deemed null in law, shall nevertheless be legitimate." 3 Hen. &
Munf. 228, n.
     4. A conclusive, presumption of legitimacy arises from marriage and
cohabitation; and proof of the mother's irregularities will not destroy this
presumption: pater est quem nuptiae demonstrant. To rebut this presumption,
circumstances must be shown which render it impossible that the husband
should be the father, as impotency and the like. 3 Bouv. Inst. n. 300-2.
Vide Bastard; Bastardy; Paternity; Pregnancy.