A lipoma is a benign tumor composed of fat (adipose) tissue. A lipoma may present as a soft, slightly movable lump or nodule underneath the skin. It is sometimes mistaken for a cyst. The cause of lipomas is not known but there are some hereditary tendencies to develop lipomas. Some lipomas may develop as the result of trauma or minor injury to the skin. If a lipoma becomes painful or grows large, it can be removed with surgery.