A gallop rhythm refers to a (usually abnormal) rhythm of the heart on auscultation. It includes three or four sounds, thus resembling the sounds of a gallop.
The normal heart rhythm contains two audible heart sounds called S1 and S2 that give the well-known "lub-dub" rhythm; they are caused by the closing of valves in the heart.
A gallop rhythm contains another sound, called S3 or S4, dependent upon where in the cycle this added sound comes.
It can also contain both of these sounds forming a quadruple gallop, and in situations of very fast heart rate can produce a summation gallop where S3 and S4 occur so close as to be indistinguishable.
Gallop rhythms may be heard in young or athletic people, but may also be a sign of serious cardiac problems like heart failure as well as pulmonary edema.