n An astringent salt, usually occurring in the form of pale crystals, much used in the dyeing and tanning trade and in certain medicines, and now understood to be a double sulphate of potassium and aluminium (K2SO4.Al2(SO4)3.24H2O). (defdate|from 14th c.)
n (chemistry) Any similar double sulphate in which either or both of the potassium and aluminium is wholly or partly replaced by other univalent or tervalent cations. (defdate|from 17th c.)