Estrogen: A female steroid hormone that is produced by the ovaries and, in lesser amounts, by the adrenal cortex, placenta, and male testes. ... An overabundance of estrogen in men causes development of female secondary sexual characteristics (feminization), such as enlargement of breast tissue.At the onset of puberty, estrogen plays a role in the development of so-called female secondary sex characteristics, such as breasts, wider hips, pubic hair and armpit hair. Estrogen also helps regulate the menstrual cycle, controlling the growth of the uterine lining during the first part of the cycle.