Nipple, supernumerary: An extra nipple. Supernumerary nipples are usually smaller than normal and vestigial (nonfunctional, without accompanying mammary glands). They tend to occur on along a roughly curved line extending from near the armpit, through the center of the normal breast, and down to the lower abdomen. This distribution is very similar to the location of nipples on those mammals that have multiple nipples along the underbelly.