Surgical removal of one or both breasts, usually done for treatment of breast cancer. Sometimes done prophylactically on women who have a strong family history of breast cancer.
In the olden days, mastectomy used to be performed crudely, cauterizing the flesh off with a hot iron. Amazon women were said to have cut their right breast off so that they could shoot arrows better as their right breast would no longer get in the way of drawing the arrow.
Nowadays, breast reconstruction using an implant, prosthesis or muscle flap is an option to lessen the cosmetic impact of this otherwise disfiguring surgical operation. It's amazing what these plastic surgeons can do.