Desmoid tumor: A benign soft-tissue tumor that does not spread to other parts of the body. Desmoid tumors occur most often in young adults, and they usually involve the limbs or trunk, but they can also arise in the abdomen or thorax. Desmoid tumors are very difficult to remove because they adhere tenaciously to surrounding structures and organs. Surgery is the treatment, but recurrence after surgery is common. Radiation therapy and limited chemotherapy have also been used. A desmoid tumor is also called aggressive fibromatosis because it is locally aggressive and fibrous, like scar tissue.