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[al-uh m]
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noun Chemistry.
  1. Also called potash alum, potassium alum. a crystalline solid, aluminum potassium sulfate, K2SO4⋅Al2(SO4)3⋅24H2O, used in medicine as an astringent and styptic, in dyeing and tanning, and in many technical processes.
  2. one of a class of double sulfates analogous to the potassium alum, as aluminum ammonium sulfate, having the general formula R2SO4⋅X2(SO4)3⋅24H2O, where R is a univalent alkali metal or ammonium, and X one of a number of trivalent metals.
  3. (not in technical use) aluminum sulfate.
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Origin of alum1

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


noun Informal.
  1. an alumna or alumnus.
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Origin of alum2

by shortening
Can be confusedalum alumna alumnae alumni alumnus (see usage note at alumnus)


  1. aluminum.
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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for alum


  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: K 2 SO 4 .Al 2 (SO 4) 3 .24H 2 O
  2. any of a group of isomorphic double sulphates of a monovalent metal or group and a trivalent metal. Formula: X 2 SO 4 .Y 2 (SO 4) 3 .24H 2 O, where X is monovalent and Y is trivalent
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Word Origin

C14: from Old French, from Latin alūmen
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for alum


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.

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Online Etymology Dictionary, © 2010 Douglas Harper

alum in Medicine


  1. 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.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

alum in Science


  1. 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.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.