noun
: a membrane rich in mucous glands ; specifically : the membrane that lines the passages and cavities of the body which communicate directly or indirectly with the exterior (as the alimentary, respiratory, and genitourinary tracts) and that consists of two chief layers of which one is a deep vascular connective-tissue stroma which in many parts of the alimentary canal contains a thin but definite layer of nonstriated muscle and the other is a superficial epithelium varying in kind and thickness but always soft and smooth and kept lubricated by the secretions of the cells and numerous glands embedded in the membrane