n. The state or quality of being firm; compactness; hardness; solidity; stability; strength; steadfastness; resoluteness; constancy; fixedness; certainty: as, the firmness of jelly; firmness of flesh; firmness of union; the firmness of a purpose; the firmness of a judge.n. In phrenology, an organ situated toward the back part of the head, between self-esteem and veneration. Its function is said to be to produce determination, constancy, and perseverance.