The lining of the womb is known as the endometrium.
Every month, the endometrium becomes thickened under the influence of oestrogen circulating in the blood. If pregnancy does not occur, the endometrial lining is shed through the neck of the womb (or cervix) and the vagina as the menses.
Endometriosis refers to the presence of endometrium outside of its normal location within the cavity of the womb. Endometriosis deposits are most often located in the pelvic cavity, for instance, on the ovaries, on the womb, between the womb and bowel (known as the Pouch of Douglas) and on the surface of the bowel. Deposits can occasionally be located at much more distant sites such as the lung.