alum
n//ˈæl.əm//alun
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). [from 14th c.]
(inorganic compound) 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. [from 17th c.]