Appendix cancer: A malignancy of the appendix, accounting for about 1 in 200 of all gastrointestinal malignancies. Although unusual, cancer of the appendix can range in type. The most common type of appendiceal cancer is carcinoid tumor with adenocarcinoma next. Tumors of the appendix often present with peritoneal seeding of the malignant cells. Advances in treatment have raised survival rates to about 80%.