pro·ges·ter·one (prō-jĕsˈtə-rōnˌ)

1. A steroid hormone secreted by the corpus luteum and by the placenta, that acts to prepare the uterus for implantation of the fertilized ovum, to maintain pregnancy, and to promote development of the mammary glands. Also called corpus luteum hormone, luteohormone, progestational hormone.
2. A drug prepared from natural or synthetic progesterone, used in oral contraceptives, hormone replacement therapy, and in the treatment of menstrual and other gynecologic disorders.