Mastitis is the inflammation of breast tissue. S. aureus is the most common etiological organism responsible, but S. epidermidis and streptococci are occasionally isolated as well. Popular usage of the term mastitis varies by geographic region. Outside the US it is commonly used for puerperal and nonpuerperal cases, in the US the term nonpuerperal mastitis is rarely used and alternative names such as duct ectasia, subareolar abscess and plasma cell mastitis are more frequently used. Chronic cystic mastitis is a different (older) name for fibrocystic disease. American usage: mastitis usually refers to puerperal (occurring to breastfeeding mothers) mastitis with symptoms of systemic infection. Lighter cases of puerperal mastitis are often called breast engorgement. In this Wikipedia article mastitis is used in the original sense of the definition as inflammation of the breast with additional qualifiers where appropriate.
Contributions by Richiez, Arcadian, and Steven Walling.