What is the primary characteristic of endometriosis?

The primary characteristic of endometriosis is the growth of endometrial tissue outside the uterus. In a normal physiological state, the endometrial tissue lines the inside of the uterus. However, in endometriosis, this tissue begins to grow on other structures such as the ovaries, fallopian tubes, the outer surface of the uterus, and other areas within the pelvic cavity. This aberrant growth can lead to inflammation, pain, and the formation of scar tissue, which are hallmark symptoms of the condition. The presence of this endometrial-like tissue outside of its normal location is what defines endometriosis and distinguishes it from other conditions related to the reproductive system.