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Also called potash alum, potassium alum. a crystalline solid, aluminum potassium sulfate, K 2 SO 4 ⋅Al 2 (SO 4) 3 ⋅24H 2 O, used in medicine as an astringent and styptic, in dyeing and tanning, and in many technical processes.
one of a class of double sulfates analogous to the , as aluminum ammonium sulfate, having the general formula R 2 SO 4 ⋅X 2 (SO 4) 3 ⋅24H 2 O, where R is a univalent alkali metal or ammonium, and X one of a number of trivalent metals.
(not in technical use) aluminum sulfate.
an alumna or alumnus.
.
Contemporary Examples

And those who take issue with the ice bucket challenge have found an unexpected spokesperson in Jackass alum Steve-O.
#IceBucketChallenge Wisdom From ‘Jackass’ Steve-O Kevin Zawacki August 20, 2014

The former president and Georgetown graduate should give novelist Virginia Commonwealth alum Tom Robbins the blues.
March Madness: Which Celebrity Alumni Will Win? Michael Solomon March 16, 2011

alum interview notes that he/she has that ‘fire in the belly’ that we are looking for.
F-Bombs and ‘Jorts’: 9 Craziest College Rejection Reasons Kristina Dell April 5, 2011

Runner up is James Fallows of The Atlantic for his hilariously pompous post “As a Harvard alum, I Apologize.”
Niall Ferguson Defends Newsweek Cover: Correct This, Bloggers Niall Ferguson August 20, 2012

One title is the first authorized Twitter book, Twitter Wit, edited by Gawker alum Nick Douglas and due out next fall.
Twitterature Isabel Wilkinson March 31, 2009

Historical Examples

Astringent fomentations; as an infusion of oak-bark, or a slight solution of alum.
Zoonomia, Vol. II Erasmus Darwin

alum is a great drier, and causes that thirst which some beer occasions; so that the more you drink of it, the more you want.
The Cook and Housekeeper’s Complete and Universal Dictionary; Including a System of Modern Cookery, in all Its Various Branches, Mary Eaton

The sands here show some fine colouring which reminds us of the more celebrated sands of alum Bay.
The Geological Story of the Isle of Wight J. Cecil Hughes

alum and copperas have been known in the Highlands long ages.
Vegetable Dyes Ethel M. Mairet

Dissolve half a pound of alum in two quarts of boiling water; then add two gallons of pure cold water.
Boy Scouts Handbook Boy Scouts of America

noun
Also called potash alum. a colourless soluble hydrated double sulphate of aluminium and potassium used in the manufacture of mordants and pigments, in dressing leather and sizing paper, and in medicine as a styptic and astringent. Formula: K2SO4.Al2(SO4)3.24H2O
any of a group of isomorphic double sulphates of a monovalent metal or group and a trivalent metal. Formula: X2SO4.Y2(SO4)3.24H2O, where X is monovalent and Y is trivalent
n.

late 14c., “whitish mineral salt used as an astringent, dye, etc.,” from Old French alum, from Latin alumen “alum,” literally “bitter salt,” cognate with Greek aludoimos “bitter” and perhaps with English ale.

alum al·um (āl’əm)
n.
Any of various double sulfates of a trivalent metal such as aluminum or iron and a univalent metal such as potassium or sodium that are used as topical astringents and styptics.
alum
(āl’əm)
Any of various crystalline double salts of a trivalent metal (such as aluminum, chromium, or iron) and a monovalent metal (such as potassium or sodium), especially aluminum potassium sulfate. Alum is widely used in industry as a hardener and purifier, and in medicine as an emetic and to stop bleeding.

noun phrase

Other such things; etcetera: baseball, apple pie, Chevrolet, and all that jazz
alumna
alumnae
alumni
alumnus
aluminum