Synonyms for firmness

noun

1. Firmness of position

   solidity, steadiness, durability, substantiality; see stability 1.

2. Firmness of material

   stiffness, impliability, hardness, toughness, solidity, density, impenetrability, durability, imperviousness, temper, impermeability, tensile strength, inflexibility, rigidity.

   Antonyms penetrability, flexibility*, fluidity.

3. Firmness of mind

   resolution, steadfastness, staunchness, strictness; see determination 2.