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alum (n.)

"whitish mineral salt used as an astringent, dye, etc.," late 14c., from Old French alum, alun, from Latin alumen "alum," literally "bitter salt," cognate with Greek aludoimos "bitter" and perhaps with English ale. The Slavic words are said to be from Germanic.

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Definitions of alum from WordNet

alum (n.)
a white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum;
Synonyms: ammonia alum / ammonium alum
alum (n.)
a white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum;
Synonyms: potassium alum / potash alum
alum (n.)
a person who has received a degree from a school (high school or college or university);
Synonyms: alumnus / alumna / graduate / grad
alum (n.)
a double sulphate of aluminum and potassium that is used as an astringent (among other things);
From wordnet.princeton.edu