With the term vexillum, in botany, we mean the largest, very colorful and showy petal that is found at the head of the corolla of the Fabaceae. The vexillum is therefore the upper petal of the papiglionaceous corolla, usually broad and erect.
The vexillum, also called banner, is the complex of the pieces of the flower or bracts, often brightly colored, suitable to recall the pollinating animals of pollination.
This function of priming the flowers of the Fabaceae is called vessillare. The banner, therefore, does not play a physiological role in the composition of the flower, but a purely attractive role.