PROGESTIN:

A hormone produced by the corpus luteum, placenta, or adrenal cortex (or synthetically manufactured) that has progesterone-like effects. Synthetic progestin is used as a contraceptive. 

Progestin is a synthetic form of progesterone usually taken with estrogen by women with a uterus because if estrogen is given alone, it can sometimes cause excessive growth of the lining of the uterus. This overgrowth may lead to a form of uterine cancer. Taking progestin with estrogen lowers the risk of developing this condition. Progestin acts similarly to progesterone, the hormone a woman's body produces before menopause that is responsible for the monthly shedding of the uterine lining, which is experienced as a menstrual period.