1. Firm, hard, solid, stiff are applied to substances that tend to retain their form unaltered in spite of pressure or force. Firm often implies that something has been brought from a yielding state to a fixed or elastic one: An increased amount of pectin makes jellies firm. Hard is applied to substances so resistant that it is difficult to make any impression upon their surface or to penetrate their interior: as hard as a stone. Solid is applied to substances that without external support retain their form and resist pressure: Water in the form of ice is solid. It sometimes denotes the opposite of hollow: a solid block of marble. Stiff implies rigidity that resists a bending force: as stiff as a poker.

Did you mean airiness?