Aestivation (in botanical terms) is the arrangement of sepals and petals in floral buds with respect to other members of whorl. It is generally confused with vernation, which is known as the arrangement of scales and new leaves in a leaf bud before it opens.
The sepals and the petals are collectively known as a perianth.
A pea flower has 5 petals. The largest petal overlaps next two lateral petals which in turn overlap the two remaining small anterior petals.
Such type of arrangement of petals is known as vexillary aestivation.