American Dictionary of the English Language

PAPILIONA'CEOUS, adjective Resembling the butterfly; a term in botany, used to describe the corols of plants which have the shape of a butterfly, such as that of the pea. The papilionaceous plants are of the leguminous kind.

The papilionaceous corol is usually four-petaled, having an upper spreading petal, called the banner, two side petals called wings, and a lower petal called the keel.