n. a steroid that includes the natural hormone progesterone and synthetic steroids (known as progestins) with physiological effects similar to those of progesterone. Progestins may be derived from progesterone or testosterone. Whereas progesterone has an antiestrogenic action, progestins may have different effects, such as proestrogenic activity. They are used in oral contraceptives, hormone replacement therapy, and medications for menstrual disorders.