pro·ges·ter·one   (prō-jěs'tə-rōn')    n.   A steroid hormone, C21H30O2, secreted by the corpus luteum of the ovary 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. A drug prepared from natural or synthetic progesterone, used in the prevention of miscarriage, in the treatment of menstrual disorders, and as a constituent of some oral contraceptives. [pro-1 + gest(ation) + (st)er(ol) + -one.]

progesterone 

Main Entry: pro·ges·ter·one
Pronunciation: prO-'jes-t&-"rOn
Function: noun
: a female steroid sex hormoneC₂₁H₃₀O₂ that is secreted by the corpus luteum to prepare the endometrium for implantation and later by the placenta during pregnancy to prevent rejection of thedeveloping embryo or fetus and that is used in synthetic forms as a birth control pill, to treat menstrual disorders, and to alleviate some cases of infertility

progesterone pro·ges·ter·one (prō-jěs'tə-rōn')
n.

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 for the prevention of miscarriage and the treatment of menstrual disorders.