Webster's English Dictionary

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alum

al.um \'al-*m\ n [ME, fr. ME alum, alun, fr. L alumen - more at ALE] 1: a 
   potassium aluminum sulfate KAl(SO4)2.12H2O or an amm onium aluminum sulfate 
   NH4Al(SO4)2.12H2O used esp. as an emetic and as an astringent and styptic 
   2: any of various double salts isomorphous with potash alum  3: aluminum 
   sulfate Al2(SO4)3