Unregulated growth. Cancer by definition is the growth in size and number in an unregulated manner of a cell line that has developed a mutation. The mutation is passed through the offspring which accumulate more and more mutations. The defining factor is that the cells act immortal as compared to normal cells that have a programmed lifespan. In the rectum this can occur in the cells that line the mucosal surface.
Rectal cancer. Rectal cancer is a tumor or growth or mass in the last part of the colon or large intestine, known as the rectum. Medically the rectum is not the opening to the outside, that is the anus. The rectum is the 12 inches or so from the anus upstream. Here is a good site that may help: http://www.Cancer.Gov/cancertopics/types/colon-and-rectal.