The Collaborative International Dictionary of English v.0.48:

Solidity \So*lid"i*ty\, n. [L. soliditas: cf. F. solidit['e].]
   1. The state or quality of being solid; density; consistency,
      -- opposed to fluidity; compactness; fullness of matter,
      -- opposed to openness or hollowness; strength;
      soundness, -- opposed to weakness or instability; the
      primary quality or affection of matter by which its
      particles exclude or resist all others; hardness;
      massiveness.
      [1913 Webster]

            That which hinders the approach of two bodies when
            they are moving one toward another, I call solidity.
                                                  --Locke.
      [1913 Webster]

   2. Moral firmness; soundness; strength; validity; truth;
      certainty; -- as opposed to weakness or
      fallaciousness; as, the solidity of arguments or
      reasoning; the solidity of principles, triuths, or
      opinions.
      [1913 Webster]

   3. (Geom.) The solid contents of a body; volume; amount of
      inclosed space.
      [1913 Webster]

   Syn: Firmness; solidness; hardness; density; compactness;
        strength; soundness; validity; certainty.
        [1913 Webster]

WordNet (r) 3.0 (2006):

solidity
    n 1: the consistency of a solid [syn: solidity, solidness]
         [ant: porosity, porousness]
    2: state of having the interior filled with matter [ant:
       hollowness]
    3: the quality of being solid and reliable financially or
       factually or morally; "the solidity of the evidence worked in
       his favor"; "the solidness of her faith gave her enduring
       hope" [syn: solidity, solidness]

Moby Thesaurus II by Grady Ward, 1.0:

174 Moby Thesaurus words for "solidity":
   abidingness, aplomb, authenticity, authoritativeness, authority,
   balance, balanced personality, body, calculability, callosity,
   callousness, changelessness, closeness, cogency, combination,
   compactness, completeness, comprehensiveness, concreteness,
   congestedness, congestion, consistence, consistency, constancy,
   cool, credit standing, crowdedness, denseness, density,
   dependability, durability, durableness, duration, durity,
   endurance, entireness, entirety, equilibrium, exhaustiveness,
   faithworthiness, fastness, firmness, fixedness, flintiness, force,
   frozenness, fusion, gluiness, hardening, hardness,
   hardness of heart, homeostasis, identity, immobility,
   immobilization, immovability, immovableness, immutability,
   impenetrability, impermeability, imperturbability, imporosity,
   inclusiveness, incompressibility, individuality, indivisibility,
   induration, inertia, inextricability, inflexibility, intactness,
   integrality, integration, integrity, invariability, invariableness,
   inveteracy, invincibility, inviolability, irreducibility,
   irremovability, jammedness, justness, lastingness, long standing,
   mass, materiality, nerve, obduracy, omnipresence, oneness,
   organic unity, palpability, particularity, permanence, permanency,
   perpetualness, persistence, persistency, pervasiveness,
   ponderability, predictability, purity, quiescence,
   relative density, reliability, resistance, restiveness, rigidity,
   rootedness, sang-froid, secureness, security, selfsameness,
   simplicity, singleness, singularity, solidarity, solidification,
   solidness, solvency, soundness, specific gravity, spissitude,
   stability, stable equilibrium, stable state, standing, stasis,
   staunchness, steadfastness, steadiness, steady nerves,
   steady state, steeliness, stoniness, stoutness, strength,
   sturdiness, substance, substantiality, substantialness,
   tangibility, temper, thickness, thoroughness, torpor, totality,
   toughness, trustworthiness, ubiquity, unchangeability,
   unchangingness, undeflectability, undividedness, unerringness,
   unification, uniformity, unindebtedness, uniqueness, unity,
   universality, univocity, unmovability, unshakable nerves,
   unshakableness, unyieldingness, validity, vis inertiae, viscidity,
   viscosity, viscousness, weight, wholeness