Medical Definition of alum
1: either of two colorless or white crystalline double sulfates of aluminum used in medicine internally as emetics and locally as astringents and styptics:a: one KAl(SO4)2·12H2O that is a sulfate of aluminum and potassium—called also potassium alumb: one consisting of an ammonium aluminum sulfate NH4Al(SO4)2·12H2O—called also ammonia alum, ammonium alum
2: any of various double salts isomorphous with potassium aluminum sulfate
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