Ejection fraction: The percentage of blood that is pumped out of a filled ventricle as a result of a heartbeat. The heart does not eject all the blood in the ventricle. Only about two-thirds of the blood is normally pumped out with each beat, and that fraction is referred to as the ejection fraction. The ejection fraction is an indicator of the heart's health. If the heart is diseased from a heart attack or another heart condition, the ejection fraction may decrease.