Quercitin n. : A yellow crystalline substance, occurring quite widely distributed in the vegetable kingdom, as is apple-tree bark, horse-chestnut leaves, etc., but originally obtained by the decomposition of quercitrin. Called also meletin.

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Taxonomy
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Defination from the web:
Quercetin is the aglycone form of a number of other flavonoid glycosides, such as rutin and quercitrin, found in citrus fruit, buckwheat and onions. Quercetin forms the glycosides quercitrin and rutin together with rhamnose and rutinose, respectively. ...