Adenomyosis

Adenomyosis is a medical condition characterized by the presence of ectopic glandular tissue found in muscle.^[1] The term adenomyosis is derived from the terms adeno- (meaning gland), myo- (meaning muscle), and -osis (meaning condition). Previously named as endometriosis interna, adenomyosis actually different from endometriosis and these two disease entities are found together in only 10% of the cases ^[2].
It usually refers to ectopic endometrial tissue (the inner lining of the uterus) within the myometrium (the thick, muscular layer of the uterus). The term "adenomyometritis" specifically implies involvement of the uterus.^[3][4]
The condition is typically found in women between the ages of 35 and 50. Patients with adenomyosis can have painful and/or profuse menses (dysmenorrhea & menorrhagia, respectively). However, because the endometrial glands can be trapped in the myometrium, it is possible to have increased pain without increased blood. (This can be used to distinguish adenomyosis from endometrial hyperplasia; in the latter condition, increased bleeding is more common.)
In adenomyosis, basal endometrium penetrates into hyperplastic myometrial fibers. Therefore, unlike functional layer, basal layer does not undergo typical cyclic changes with menstrual cycle ^[5].
Adenomyosis may involve the uterus focally, creating an adenomyoma. With diffuse involvement, the uterus becomes bulky and heavier.

Source: Wikipedia