An umbo is a raised area in the center of a mushroom cap

Pileus (mycology)

The pileus is the technical name for the cap, or cap-like part, of a basidiocarp or ascocarp that supports a spore-bearing surface, the hymenium. The hymenium may consist of lamellae, tubes, or teeth, on the underside of the pileus...

. Caps that possess this feature are called umbonate. Umbos that are sharply pointed are called acute, while those that are more rounded are broadly umbonate. If the umbo is elongated, it is cuspidate, and if the umbo is sharply delineated but not elongated (somewhat resembling the shape of a female breast), it is called mammilate or papillate.