Adenomyosis is defined as:

Adenomyosis is characterized by the presence of endometrial tissue that has invaded the myometrium, which is the muscular layer of the uterine wall. This condition can lead to a range of symptoms, including menstrual pain, heavy menstrual bleeding, and an enlarged uterus. Understanding adenomyosis as endometrial tissue located within the myometrium distinguishes it from other conditions related to the endometrium, such as endometriosis, where endometrial tissue is found outside the uterus. The definition reflects the internal disruption within the uterine wall rather than the presence of tissue in abnormal locations or the impacts on bleeding patterns alone.