1 [al-uhm]
noun Chemistry.
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 potassium alum, 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.

1275–1325; Middle English < Anglo-French < Latin alūmen; replacing Old English alefne, ælifnæ < Old Welsh (compare MWelsh elyf) < Latin alūmini- (stem of alūmen) Unabridged


2 [uh-luhm]
noun Informal.
an alumna or alumnus.

by shortening

alum, alumna, alumnae, alumni, alumnus (see usage note at alumnus).


Chemistry. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
alum (ˈæləm)
1.  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
2.  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
[C14: from Old French, from Latin alūmen]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

early 14c., "whitish mineral salt used as an astringent, dye, etc.," from O.Fr. alum, from L. alumen "alum," lit. "bitter salt," cognate with Gk. aludoimos "bitter" and Eng. ale.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

alum al·um (āl'əm)
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.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
alum   (āl'əm)  Pronunciation Key 
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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Abbreviations & Acronyms
  1. alumna

  2. alumnae

  3. alumni

  4. alumnus

The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Soak in alum water to cover, allowing two teaspoons powdered alum to each quart of water.
Cover with alum water, allowing two teaspoons powdered alum to each quart of water.
Alum treatment in shallow lakes for phosphorus inactivation may last for eight or more years.
Alum treatment doubled the volume of anaerobically digested sludge produced.
Image for alum
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