endocardium

noun, plural en·do·car·di·a [en-doh-kahr-dee-uh] /ˌɛn doʊˈkɑr di ə/. Anatomy.

1. the serous membrane that lines the cavities of the heart.

noun plural -dia (-dɪə)

1. the membrane that lines the cavities of the heart and forms part of the valves

n. pl. en•do•car•di•a (-dē-ə)

1. The thin serous membrane, composed of endothelial and subendothelial tissue, that lines the interior of the heart.