1. A circumscribed pouch or sac of variable size occurring normally or created by herniation of the lining mucous membrane through a defect in the muscular coat of a tubular organ.
2. A sac or pouch in the walls of a canal or organ.
3. A pouch or sac opening from a tubular or saccular organ; such as, the gut or bladder.

The remains of the yolk stalk of the embryo, which, when persisting abnormally as a blind sac or pouch in the adult, is located on the ileum a short distance above the cecum; it may be attached to the umbilicus and, if the lining includes gastric mucosa, peptic ulceration and bleeding may result.