WordNet sense 1 (the state in which a substance has no tendency to flow under moderate stress; resists forces (such as compression) that tend to deform it; and retains a definite size and shape):

solid

WordNet sense 2 (the quality of being substantial or having substance):

substantiality, substantialness

WordNet sense 3 (the quality of being solid and reliable financially or factually or morally):

solidity