Mucus is a slippery secretion of the lining of various membranes in the body (mucous membranes). Mucus aids in the protection of the lungs by trapping foreign particles that enter the nose during normal breathing. Additionally, it prevents tissues from drying out.

Mucus is made by goblet cells in the mucous membranes that cover the surfaces of the membranes. It is made up of mucins and inorganic salts suspended in water.

Mucus performs two tasks. It is used as a lubricant for materials which must pass over membranes for example food passing down the esophagus. Also it catches unknown matter and tries to stop it from entering the body.

Mucus is a viscous colloid containing antiseptic enzymes (such as lysozyme) and immunoglobulins.

Increased mucus production in the respiratory tract is a symptom of many common diseases, such as the common cold. The presence of mucus in the nose and throat is normal, but increased quantities can impede comfortable breathing and must be cleared by blowing the nose or expectorating phlegm from the throat. Among the components of nasal mucus is tears.