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 algae and other plants.