A mastectomy is an operation to remove the whole breast, usually because it has been affected by cancer. If only part of the breast tissue needs to be removed, it is known as a segmentectomy or segmental mastectomy. If just a lump is removed, it is called a lumpectomy.
A mastectomy is sometimes done on healthy breasts that are not cancerous to reduce the risk of breast cancer. This is known as risk-reducing (prophylactic) mastectomy.
Definition from NHS Choices: mastectomies