alum (Noun) — A white crystalline double sulphate of aluminium: the ammonium double sulphate of aluminium.
alum (Noun) — A white crystalline double sulphate of aluminium: the potassium double sulphate of aluminium.
alum (Noun) — A person who has received a degree from a school (high school, college or university).
alum (Noun) — A double sulphate of aluminium and potassium that is used as an astringent (among other things).