1. A temporary opening between the developing ventricles of the heart. The ventricles arise as a single cavity is divided by the developing interventricular septum. Before the septum closes completely, the remaining opening between the two ventricles is termed the interventricular foramen. (wikipedia.org)
2. The small opening (on both the right and left sides) that connects the third ventricle in the diencephalon with the lateral ventricle in the cerebral hemisphere. (wordnetweb.princeton.edu)
Also called primary interventricular foramen.