1. Any of several steroid hormones, such as estradiol and estrone, that are produced primarily by the ovaries, stimulate the development and maintenance of female secondary sex characteristics, exert systemic effects such as the growth and maturation of long bones, and promote estrus in many female mammals. Estrogens synthesized from plant sources or obtained from horses are used as drugs, primarily to treat estrogen deficiency.

2. Any of several synthetic compounds that mimic the physiologic activity of estrogen, such as ethinyl estradiol, used primarily in oral contraceptives.