A small projecting body part similar to a nipple in form: as a : a vascular process of connective tissue extending into and nourishing the root of a hair, feather, or developing tooth b : any of the vascular protuberances of the dermal layer of the skin extending into the epidermal layer and often containing tactile corpuscles c : renal papilla d : any of the small protuberances on the upper surface of the tongue.