A type of soft tissue tumor that forms in fibrous (connective) tissue, usually in the arms, legs, or abdomen. It may also occur in the head and neck. Desmoid tumors are usually benign (not cancer). They often recur (come back) after treatment and spread to nearby tissue, but they rarely spread to other parts of the body. They may occur in adults or children. Also called aggressive fibromatosis and desmoid-type fibromatosis.