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)