The Green-veined White (Pieris napi) is a butterfly of the Pieridae family.
A circumboreal species widespread across Europe and Asia, including the Indian subcontinent, Japan and the Maghreb and North America. (Many authors consider the Mustard White of North America to be conspecific with napi or consider napi to be a superspecies). It is found in meadows, hedgerows and woodland glades but not as often in gardens and parks like its close relatives the Large and Small Whites, for which it is often mistaken. Like other "white" butterflies, the sexes differ. The female has two spots on each forewing, the male only one. The veins on wings of the female are usually more heavily marked. The underside hindwings are pale yellow with the veins highlighted by black scales giving a greenish tint, hence Green-veined White. Unlike these two butterflies it rarely chooses garden cabbages to lay its eggs on, preferring wild crucifers.
Description from Wikipedia: http://en.wikipedia.org/wiki/Green-veined_White
Bit of plant removed from right side of image in ACR