osteosarcoma Syllabification: (os·te·o·sar·co·ma)Pronunciation: /ˌästēōsärˈkōmə/Definition of osteosarcoma noun (plural osteosarcomas or osteosarcomata /-mətə/) Medicine a malignant tumor of bone in which there is a proliferation of osteoblasts. osteosarcoma in other Oxford dictionaries Definition of osteosarcoma in the British & World English dictionary