A breast cyst is a fluid-filled sac that develops in the breast tissue. It can vary in size and shape and may be painful or tender to the touch. There are several synonyms for the term breast cyst that include mammary cyst, breast lump, cystic breast lesion, and fibrocystic breast disease. While breast cysts are typically benign and may not require treatment, they can cause concern and anxiety for those affected. It is important to seek medical attention if you notice any changes or abnormalities in your breast tissue to ensure proper diagnosis and treatment.