NCI: An endometrioid adenocarcinoma arising from the ovary. It comprises 10% to 25% of all primary ovarian carcinomas. Grossly, endometrioid carcinoma may present as a cystic or solid mass. Microscopically, the tumor greatly resembles the appearance of the ordinary type of endometrial adenocarcinoma. As a group, endometrioid carcinoma has a prognosis twice as good as that of serous or mucinous carcinoma.