Papilionaceous is a flower from the Papilionaceae family (as in the bean or pea) with a corolla, usually with five petal

Petal

Petals are modified leaves that surround the reproductive parts of flowers. They often are brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals lying...

s that include a large upper petal, the vexillum(standard petal), enclosing two lateral wings resembling a butterfly and a lower keel

Keel

In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

 petals which are united.The charecteristic aestivation of this flower is vexillary imbricate.