Meaning of “alum” in the English Dictionary

british dictionary

"alum" in British English

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alumnoun [ U ]

uk /ˈæl.əm/ us /ˈæl.əm/ specialized

a chemical substance containing aluminium used in dyeing (= changing the colour of something) and as an astringent (= substance that causes skin to tighten)

(Definition of “alum” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)