Endometriosis is defined as the presence of endometrial-like tissue outside the uterus, leading to a chronic inflammatory tissue reaction, pain, and infertility. It is most often found on: ovaries, fallopian tubes, tissue that holds the uterus in place (ligaments), the outer surface of the uterus, but sometimes it can also be found on the vagina, cervix, bowel, bladder, rectum.

It is a relatively common diagnosis, but unrecognized. Just as the endometrium inside the uterus has a menstrual cycle, so the endometrium outside the uterus goes through the same phases. The endometrium outside the uterine cavity desquamates inside the organ on which it is located, causing irritation, inflammation and later adhesions. The adhesions enable bonding between the surrounding tissue to the uterus, thus reducing its function.

Endometriotic cysts on the ovary are also known as chocolate cysts.