1. The organ in mammals and other viviparous animals by means of which the embryo is attached to the wall of the uterus. It is composed of embryonic and maternal tissues: extensions of the chorion and allantois grow into the uterine wall so that materials (e.g. oxygen, nutrients, and metabolic waste products) can pass between the blood of the embryo and its mother. There is, however, no direct connection between the maternal and embryonic blood: blood travels to the placenta from the embryo through the arteries of the umbilical cord and returns via the umbilical vein. The placenta is eventually expelled as part of the afterbirth.

2. A ridge of tissue on the ovary wall of flowering plants to which the ovules are attached. The arrangement of ovules on the placenta (placentation) is variable, depending on the number of carpels and whether they are free (see apocarpy) or fused (see syncarpy).