A bolt with a hexagonal socket in its head that is designed to be used with an Allen wrench The Allen bolt is integral to the crank and remains in it when the crank is removed from the spindle. The terms "hex wrench" and "Allen wrench" refer to the same tool: an L-shaped hexagonal device designed to fit and drive bolts and screws that have a hexagonal socket in their heads.
Allen keys protect against the unintentional stripping of fasteners. Since they are used with hexagonal fasteners, they'll “grip” fasteners better than other common screwdrivers and wrenches. This strong gripping action protects fasteners from being stripped during installation or removal.