A bipinnate, or bipinnately compound, leaf is divided twice: each leaflet is subdivided into smaller leaflets. The pinna (pl. pinnae) is the name of the first division. The pinnule is the name of the ultimate division. The pinnae are on the primary rachis, while the pinnules are on the secondary rachis.
Image Above: A bipinnately compound leaf of Delonix regia, royal poinciana.
Definition adapted from and image from URL: http://www.life.illinois.edu/help/digitalflowers/Vegetative/47.htm