1. A structure formed on the wall of the uterus during pregnancy which serves to exchange oxygen, wastes and nutrients between the mother's blood and the embryo/fetus' blood; the placenta is expelled as afterbirth.

    There are several types of placentas:

  2. In plants: a region, within an ovary, to which ovules are attached.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.