A colon polyp is a growth on the inner lining of the colon (large intestine) or rectum. Polyps in the colon arise from uncontrolled multiplication of cells leading to growth of tissue. This can occur due to mutation in the genes of the cells. A polyp can vary in size from few millimeters to several centimeters. Polyps look like mushrooms and can have a variety of shapes. They can flat, slightly raised, or have a stalk.