An extra, clearly heard heart sound which, when the heart rate is fast, resembles a horse's gallop. It occurs in heart failure but is also heard in many normal children.
A gallop rhythm refers to a (usually abnormal) rhythm of the heart on auscultation. The normal heart rhythm contains two audible sounds, called S1 and S2, giving the well-known "lub-dub" rhythm. They are caused by the closing of valves in the heart.