A common, benign condition of the uterus in which the endometrium (the inner uterus) grows into the adjacent myometrium (the uterine musculature located just outside the endometrium). The myometrium may respond to this intrusion with muscular overgrowth. If an island of endometrial tissue is contained within the myometrium, it forms an adenomyoma. Also known as endometriosis interna, endometriosis uterina, adenomyosis uteri, and adenomyometritis.