Placenta accreta: The abnormal adherence of the chorionic villi (vascular fingers of the chorion, a part of the placenta) to the myometrium (the muscle of the uterus), . Normally there is tissue intervening between the chorionic villi and the myometrium but in placenta accreta, these vascular processes of the chorion grow directly in the myometrium. Placenta accreta can progress into placenta percreta.