The endometrium is the inner lining of the uterus.
If conception takes place, the embryo implants into the endometrium. If pregnancy does not occur, the endometrium sheds in a process commonly known a menstruation. Each month, the endometrium renews, preparing for the potential of pregnancy, and then sheds if conception doesn't happen.
Endometriosis is a condition in which endometrium is found outside the uterine cavity.