Breast infection: Infection of breast tissue usually caused by bacteria, most often staph (Staphylococcus aureus), which are found on the skin and enter the breast through a break in the skin or nipple, as during breast-feeding. The infected breast may be swollen, hot, reddened, and painful. There may be low grade fever. Treatment includes warm wet compresses and antibiotics.

The term mastitis is often used as a synonym for breast infection but, strictly speaking, mastitis means inflammation of the breast irrespective of the cause.