Symmastia is defined as a confluence of the breast tissue of both breasts across the midline anterior to the sternum. It can be surgically corrected by a plastic surgeon.
Symmastia can either be a congenital anomaly or iatrogenic. Congenital symmastia is a rare condition with few published cases. Iatrogenic symmastia may occur following breast augmentation, forming what is also colloquially referred to as a "uniboob".