A viscid fluid secreted by mucous membranes, which it serves to moisten and protect. It covers the lining membranes of all the cavities which open externally, such as those of the mouth, nose, lungs, intestinal canal, urinary passages, etc.

Any other animal fluid of a viscid quality, as the synovial fluid, which lubricates the cavities of the joints; -- improperly so used.

A gelatinous or slimy substance found in certain algæ and other plants.

The sticky, slippery material released by mucous membranes and glands. In the lungs, mucus serves as a defense mechanism, trapping bacteria and foreign particles so that they can be moved out of the lungs.