1. A steroid hormone, C₂₁H₃₀O₂, that is secreted by the corpus luteum of the ovary and by the placenta and that acts to prepare the uterus for implantation of the fertilized ovum and to maintain pregnancy.

2. This compound formulated as a drug, usually prepared synthetically from phytosterols, used in the treatment of infertility, amenorrhea, abnormal uterine bleeding, and certain other conditions.