The appendix is a narrow tube-shaped sac that connects to the colon near the beginning of the large intestines. It is a part of the gastrointestinal tract that absorbs nutrients and eliminates waste from the body. 

Appendix cancer, also known as appendiceal cancer, is a rare cancer that develops when healthy cells in the appendix grow and multiply abnormally. These cancerous cells become a mass or tumour inside the appendix, which is often found by accident after surgical removal of the appendix.