atrioventricular (AV) septum

a·tri·o·ven·tric·u·lar (AV) sep·tum

(ā'trē-ō-ven-trik'yū-lăr sep'tŭm) [TA] The small part of the membranous septum of the heart just above the septal cusp of the tricuspid valve that separates the right atrium from the left ventricle.