Basicity of acid — is the number of hydrogen ions [H+] which can be produced per molecule of the acid in aq. soln. Acetic acid [CH3COOH] ionises in aq. soln. and gives one hydrogen ion per molecule of the acid, hence acetic acid is monobasic i.e., it’s basicity is one.
Acidity of base — is the number of hydroxyl ions [OH–] which can be produced per molecule of the base in aq. soln. Calcium hydroxide Ca(OH)2 ionises in aq. soln. and gives two hydroxyl ions per molecule of the base, hence calcium hydroxide is a diacidic base i.e., it’s acidity is two.