Papilionaceous: Butterfly-like, as the irregular corolla of a pea, with a banner petal, two wing petals, and two fused keel petals.*
Baptisia is a member of the pea family (Fabaceae). It has classic papilionaceous flowers.
*Harris, J.G., Harris, M.W. (2001). Plant identification terminology: An illustrated glossary (2nd ed.). Spring Lake: Spring Lake Publishing.