Zenker's diverticulum is a medical condition characterized by the formation of a pouch or sac in the upper part of the esophagus, specifically at the junction of the pharynx and esophagus. This pouch can cause difficulty in swallowing, regurgitation of undigested food, and bad breath. It occurs due to a dysfunction in the muscles of the upper esophageal sphincter, leading to increased pressure and subsequent formation of the diverticulum.