the growth of endometrial tissue in abnormal locations, as on the ovaries or within the peritoneal cavity
See endometriosis in American Heritage Dictionary 4
(ĕnˌdō-mēˌtrē-ōˈsĭs)
noun
A condition, usually resulting in pain and dysmenorrhea, that is characterized by the abnormal occurrence of functional endometrial tissue outside the uterus.