What Is an Endometrioma?
Endometriomas are cysts that can form when tissue from the lining of the uterus grows in the ovaries. They are part of a condition known as endometriosis. Endometriomas are also called "chocolate cysts" because they are filled with dark brown menstrual blood. When you have endometriosis, endometrial tissue grows outside the uterus and into other areas of the abdomen/pelvic region.
Source:Verywell Health