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. That which hinders the approach of two bodies when they are moving one toward another, I call solidity. (locke)
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.
Origin: L. Soliditas: cf. F. Solidite.