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]