Breast lump: A localized swelling, knot, bump, bulge or protuberance in the breast. Breast lumps may appear in both sexes at all ages. In women, the fear is usually of breast cancer but many breast lumps turn out, fortunately, to be due to benign conditions that can be successfully treated such as infection, trauma, fibroadenoma, cyst, or fibrocystic condition of the breast. However, no breast lump should be dismissed as benign until it has been checked by a physician.