Giant cell tumor of bone: A tumor of bone characterized by massive destruction of the end (epiphysis) of a long bone. The site most commonly involved by this tumor is the knee, both the far end of the femur and the near end of the tibia. The term 'giant cell' reflects the fact that microscopic analysis of the tumor reveals large multinucleate cells (cells with more than one nucleus). The tumor is often coated by new bony growth. It causes pain, restricts movement, and is usually malignant. Treatment is by surgery, usually followed by chemotherapy.