What is defined by the presence of endometrial tissue outside the uterus?

The presence of endometrial tissue outside the uterus is characteristic of endometriosis. This condition involves the growth of tissue similar to the lining inside the uterus (the endometrium) in areas outside the uterine cavity, including the ovaries, fallopian tubes, and the pelvic lining. This aberrant tissue can cause significant pain, especially during menstrual cycles, as well as other symptoms such as infertility and pelvic adhesions.

In contrast, polycystic ovarian syndrome involves hormonal imbalances and the presence of small cysts on the ovaries, but it does not entail endometrial tissue located outside the uterus. Fibroids, or uterine leiomyomas, refer to non-cancerous growths found within the uterine wall but do not involve ectopic endometrial tissue. Pelvic inflammatory disease is an infection of the reproductive organs, typically caused by sexually transmitted infections, and it is not defined by the presence of endometrial tissue outside the uterus.

Thus, the defining feature of endometriosis is the ectopic presence of endometrial-like tissue, making it the correct answer.