Definition - What does Chorion mean?
The chorion is the outermost layer of the amniotic sac, the bag of fluid that protects the embryo, and later the fetus, during pregnancy. In early stages of pregnancy the chorion develops finger like extensions called villi which later transitions into the placenta.
FertilitySmarts explains Chorion
The amniotic sac is made up of membranes, which are porous layers of cells that together form a structure. There are two sections within the amniotic sac, the outer later, called the chorion, and the inner layer, called the amnion. The chorion is the layer that divides the fetus from the uterine cavity and wall.