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Urtica (nettle; family Urticaceae) A genus of annual or perennial herbs with ridged stems, opposite, simple leaves, and numerous stinging hairs. The flowers are small, green, and unisexual, in clusters along spike-like stems in leaf axils, and both sexes of flowers may occur on the same plant, or the plants may be dioecious. There are 45 species, mostly found in temperate regions.