esculin
noun
es·cu·lin
variants:
or aesculin \ ˈes-kyə-lən 
\
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Medical Definition of esculin
: a glucoside C15H16O9 from the inner bark of the horse chestnut (Aesculus hippocastaneum) and roots of the yellow jessamine (Gelsemium sempervirens) that absorbs ultraviolet rays