Ulcerative Colitis is a condition that causes inflammation and ulceration of the inner lining of the rectum and colon (the large bowel). In UC, ulcers develop on the surface of the lining and these may bleed and produce mucus.

The inflammation usually begins in the rectum and lower colon, but it may affect the entire colon. If UC only affects the rectum, it is called proctitis, while if it affects the whole colon it may be called total colitis or pancolitis.
https://www.crohnsandcolitis.org.uk/about-inflammatory-bowel...