The Collaborative International Dictionary of English v.0.48:

Validity \Va*lid"i*ty\, n. [Cf. F. validit['e], L. validitas
   strength.]
   [1913 Webster]
   1. The quality or state of being valid; strength; force;
      especially, power to convince; justness; soundness; as,
      the validity of an argument or proof; the validity of an
      objection.
      [1913 Webster]

   2. (Law) Legal strength, force, or authority; that quality of
      a thing which renders it supportable in law, or equity;
      as, the validity of a will; the validity of a contract,
      claim, or title.
      [1913 Webster]

   3. Value. [Obs.] "Rich validity." --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

validity
    n 1: the quality of being valid and rigorous [syn: cogency,
         validity, rigor, rigour]
    2: the quality of having legal force or effectiveness [syn:
       validity, validness]
    3: the property of being strong and healthy in constitution
       [syn: robustness, hardiness, lustiness, validity]

Moby Thesaurus II by Grady Ward, 1.0:

148 Moby Thesaurus words for "validity":
   actionability, admissibility, admissibleness, advantageousness,
   agreeableness, allowableness, amperage, applicability, armipotence,
   auspiciousness, authenticity, authoritativeness, authority, beef,
   beneficialness, benevolence, benignity, black power, brute force,
   calculability, charge, charisma, class, clout, cogence, cogency,
   compulsion, constitutional validity, constitutionalism,
   constitutionality, dependability, desert, dint, drive, due process,
   duress, effect, effectiveness, effectuality, efficacy, energy,
   excellence, expedience, fairness, faithworthiness, favorableness,
   fineness, firmness, first-rateness, flower power, force,
   force majeure, forcefulness, full blast, full force, goodliness,
   goodness, grace, gravity, healthiness, helpfulness, influence,
   invincibility, jurisdiction, justice, justiciability,
   justifiableness, justness, kindness, lawfulness, legal form,
   legal process, legalism, legality, legitimacy, legitimateness,
   licitness, main force, main strength, mana, merit, might,
   might and main, mightiness, moxie, muscle power, niceness,
   permissibility, permissibleness, pizzazz, pleasantness, poop,
   potence, potency, potentiality, power, power pack, power structure,
   power struggle, powerfulness, predictability, prepotency,
   productiveness, productivity, profitableness, puissance, pull,
   punch, push, quality, reliability, rewardingness, rightfulness,
   sanctionableness, scope, secureness, security, sinew, skillfulness,
   solidity, soundness, stability, staunchness, steadfastness,
   steadiness, steam, strength, strong arm, substantiality,
   superiority, superpower, trustworthiness, unerringness, usefulness,
   value, vehemence, vigor, vim, virility, virtue, virtuousness,
   virulence, vitality, warrantableness, wattage, weight, wholeness,
   worth