Colon cancer: A malignant tumor arising from the inner wall of the large intestine (the colon). In the US, colon cancer is the third leading type of cancer in males and the fourth in females. Risk factors for cancer of the colon and rectum (colorectal cancer) include colon polyps, long-standing ulcerative colitis, and genetic family history. Most colorectal cancers develop from polyps. Removal of colon polyps can prevent colorectal cancer. Colon polyps and early colon cancer can have no symptoms. Therefore, regular screening is important, starting at age 50 (or earlier, if added risk factors are present). Diagnosis can be made by barium enema or by colonoscopy, with biopsy confirmation of cancer tissue. Surgery is the most common treatment for colorectal cancer.