AMBROSIA, in botany, a genus of the monœcia pentandria class. The male floscules of the ambrosia have no petals; the fruit of the female is prickly, and shaped like a club, containing one oblong seed in each. There are four species of ambrosia, viz. the trifida, ciliator, and the artemifolia, all natives of America; and the maritima, a native of Greece.