(1) The moist inner surface that lines the mouth, nasal sinuses, stomach, intestines and many other parts of the body. It secretes mucous which acts as a protective barrier and lubricant, as well as a medium for carrying enzymes.
(2) Any tissue lining body cavities and canals which come in contact with the air, and kept moist by secretions of various types of glands (eg, inside the mouth).
(3) Any of the membranes lining the passages of the body, such as the respiratory and digestive tracts, that open to the outside. Cells in the mucous membranes secrete mucus, which lubricates the membranes and protects against infection.