Lipoma: A benign tumor composed of fat cells (ADIPOCYTES). It can be surrounded by a thin layer of connective tissue (encapsulated), or diffuse without the capsule.Neoplasms, Adipose Tissue: Neoplasms composed of fatty tissue or connective tissue made up of fat cells in a meshwork of areolar tissue. The concept does not refer to neoplasms located in adipose tissue.Lipomatosis, Multiple Symmetrical: A condition characterized by the growth of unencapsulated masses of ADIPOSE TISSUE symmetrically deposited around the neck, shoulders, or other sites around the body.Liposarcoma: A malignant tumor derived from primitive or embryonal lipoblastic cells. It may be composed of well-differentiated fat cells or may be dedifferentiated: myxoid (LIPOSARCOMA, MYXOID), round-celled, or pleomorphic, usually in association with a rich network of capillaries. Recurrences are common and dedifferentiated liposarcomas metastasize to the lungs or serosal surfaces. (From Dorland, 27th ed; Stedman, 25th ed)Lipomatosis: A disorder characterized by the accumulation of encapsulated or unencapsulated tumor-like fatty tissue resembling LIPOMA.