A mastectomy is a type of surgery that removes breast tissue. There are a few different types of mastectomies that can be performed depending on your age, breast tissue, tumour size and location and the presence of cancer in surrounding tissue and lymph nodes. The aim of a mastectomy is to remove the tumour (either wholly or partially), and surrounding skin that could possibly have cancer cells in them, to prevent cancer from returning or spreading.
Another term you may have heard of is a lumpectomy, which is where the tumour is removed and a small amount of surrounding tissue. Technically a lumpectomy is a type of mastectomy.