noun.
The thin serous membrane, composed of endothelial tissue, that lines the interior of the heart.
The Century Dictionary and Cyclopedia
noun.
In anatomy, the lining of the heart, as distinguished from the pericardium, or investing membrane of that organ; the membrane forming the inner surface of the walls of the cardiac cavities, or this surface itself.
the GNU version of the Collaborative International Dictionary of English
noun.
The membrane lining the cavities of the heart.
noun.
A thin serous membrane that lines the interior of the heart.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
noun.
the membrane that lines the cavities of the heart and forms part of the heart valves
Word Usage
"The endocardium is a thin, smooth membrane which lines and gives the glistening appearance to the inner surface of the heart; it assists in forming the valves by its reduplications, and is continuous with the lining membrane of the large bloodvessels."