alum

Short definition from Wiktionary

Noun

1. 1
   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).
2. 2
   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.
3. 3
   A past attendee or graduate (of any gender) of a college, university or other educational institution.

Verb

1. 1
   To steep in, or otherwise impregnate with, a solution of alum; to treat with alum.