Chondrosarcoma: A malignant tumor that forms in cartilage cells (chondroplasts) and that produces cartilage matrix. Chondrosarcoma can be primary or secondary. Primary chondrosarcoma forms in bone and is found in children. Secondary chondrosarcoma arises from preexisting benign defects of cartilage (such as an osteochondroma or enchondroma), usually after age 40. Treatment is mainly by surgery.