/ˌendōˈkärdēəm/
Noun, pl. endocardia
1. The innermost layer of tissue that lines the chambers of the heart. It underlies the much more voluminous myocardium, the muscular tissue responsible for the contraction of the heart. (wikipedia.org)
2. A thin serous membrane that lines the interior of the heart. (wiktionary.org)
3. The thin, smooth membrane that lines the inside of the chambers of the heart and forms the surface of the valves. (Google Dictionary)
Word origin: endo– + G. kardia, heart