alum
noun [ U ] chemistry specialized uk/ˈæl.əm/ us/ˈæl.əm/
(chemistry)
明矾,矾(含铝的化学物质,作为染色剂或收敛剂)
a chemical substance containing aluminium used in dyeing (= changing the colour of something) and as an astringent (= substance that causes skin to tighten)