A diverticulum is a hollow outpouching or herniation of part of the bladder wall. The component parts of a true diverticulum are a mouth, neck and sac which have may a capacity up to 2,500 c.c.
Reference: Berry N, Schneiderman C. Vesical Diverticulum. Can Med Assoc J. 1948 February; 58(2): 129–133.
Credit: Dr Piotr Polaków