alum

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of various double sulfates of a trivalent metal such as aluminum, chromium, or iron and a univalent metal such as potassium or sodium, especially aluminum potassium sulfate, AlK(SO₄)₂·12H₂O, widely used in industry as clarifiers, hardeners, and purifiers and medicinally as topical astringents and styptics.
2. n. Informal An alumna or alumnus.

Century Dictionary and Cyclopedia

1. n. The general name of a class of double sulphates formed by the union of aluminium, iron, chromium, or manganese sulphate with the sulphate of some other metal, commonly an alkaline metal or ammonium. Common or potash alum has the formula Al₂(SO₄)₃ + K₂SO₄ + 24H₂O. It is produced by mixing concentrated solutions of potassium sulphate and crude aluminium sulphate. The double salt at once crystallizes in octahedrons. Alum is soluble in water, has a sweetish-sour taste, reddens litmus, and is a powerful astringent. In medicine it is used internally as an astringent, externally as a styptic applied to severed blood-vessels. In the arts it is used as a mordant in dyeing, and extensively in other ways. When mixed in small amount with inferior grades of flour, it is said to whiten them in the process of bread-making, but its effect on the system is injurious.
2. To steep in or impregnate with a solution of alum.

Wiktionary

1. n. the double sulphate of potassium and aluminium with chemical formula K2SO4.Al2(SO4)3.24H2O.
2. n. 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. n. A graduate of a university or other institution.

GNU Webster's 1913

1. n. A double sulphate formed of aluminium and some other element (esp. an alkali metal) or of aluminium. It has twenty-four molecules of water of crystallization.
2. v. To steep in, or otherwise impregnate with, a solution of alum; to treat with alum.

WordNet 3.0

1. n. a white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum
2. n. a double sulphate of aluminum and potassium that is used as an astringent (among other things)
3. n. a white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum
4. n. a person who has received a degree from a school (high school or college or university)