Catarrh ( /kəˈtɑr/) is a disorder of inflammation of the mucous membranes in one of the airways or cavities of the body. It can result in a thick exudate of mucus and white blood cells caused by the swelling of the mucous membranes in the head in response to an infection. It is a symptom usually associated with the common cold and chesty coughs, but can also be found in patients with infections of the adenoids, middle ear, sinus or tonsils. The phlegm produced by catarrh may either discharge or cause a blockage which may become chronic.

A catarrh blockage may result in discomfort with (and what is known as ear fear of):

and other activities associated with a change in pressure.

Even the shallow end of a swimming bath can be troublesome; barotrauma -- a problem linked to head pressure changes which is affected by catarrh blockages—can occur in as little as 4 feet (1.2 m) of water depth.

The word "catarrh" comes from the Greek "katarrhein": kata- meaning "down" and rhein meaning "to flow."