Symmastia is a rare congenital anomaly in which soft tissue connects, or webs, both breasts across the midline anterior to the sternum. It can be surgically corrected by a plastic surgeon.

The term is also very commonly used in plastic surgery

Plastic surgery

Plastic surgery is a medical specialty concerned with the correction or restoration of form and function. Though cosmetic or aesthetic surgery is the best-known kind of plastic surgery, most plastic surgery is not cosmetic: plastic surgery includes many types of reconstructive surgery, hand...

 to describe a similar appearance that occurs undesirably following breast augmentation

Breast augmentation

Breast augmentation denotes the breast implant and fat-graft mammoplasty procedures for correcting the defects, and for enhancing the size, form, and feel of a woman’s breasts...

. The colloquial term used to describe this is the "uniboob".