A reaper is a farm implement or person that reaps (cuts and often also gathers) crops at harvest when they are ripe. Usually the crop involved is a cereal grass. The first documented reaping machines were Gallic reapers that were used in Roman times in what would become modern-day France. The Gallic reaper involved a comb which collected the heads, with an operator knocking the grain into a box for later threshing.

Most modern mechanical reapers cut grass; most also gather it, either by windrowing or picking it up. Modern machines that not only cut and gather the grass but also thresh its seeds (the grain), winnow the grain, and deliver it to a truck or wagon, called combine harvesters or simply combines, which are the engineering descendants of earlier reapers.

Hay is harvested somewhat differently from grain; in modern haymaking, the machine that cuts the grass is called a hay mower or, if integrated with a conditioner, a mower-conditioner. As a manual task, cutting of both grain and hay may be called reaping, involving scythes, sickles, and cradles, followed by differing downstream steps. Traditionally all such cutting could be called reaping, although a distinction between reaping of grain grasses and mowing of hay grasses has long existed; it was only after a decade of attempts at combined grain reaper/hay mower machines (1830s to 1840s) that designers of mechanical implements began resigning them to separate classes.

Mechanical reapers substantially changed agriculture from their appearance in the 1830s until the 1860s through 1880s, when they evolved into related machines, often called by different names (self-raking reaper, harvester, reaper-binder, grain binder, binder), that collected and bound the sheaves of grain with wire or twine.