A viscid fluid secreted by the mucous membrane, which it serves to moisten and defend. It covers the lining membranes of all the cavities which open externally, such as those of the mouth, nose, lungs, intestinal canal, urinary passagesIt differs from gelatine. In the action of chewing, the mucus mixeth with the ailment.

This term has also been applied to other animal fluids of a viscid quality, as the synovial fluid, which lubricates the cavities of the joints.