1. Enlargement of the male breast, in a minority of cases accompanied by galactorrhea.

In a few cases, breast development is so marked as to require corrective mastectomy.

2. Excessive development of the male mammary glands, due mainly to ductal proliferation with periductal edema; frequently secondary to increased estrogen levels, but mild gynecomastia may occur in normal adolescence.
3. Also called mammary feminism (older term), gynecomasty (seldom used), gynecomastism (rarely used), gynecomazia (rarely used).