A cotter is a pin or wedge passing through a hole to fix parts tightly together. In British usage cotter pin has the same meaning, but in U.S. it refers to different fasteners.
Formerly, it was common to mount bicycle crank arms using a cotter, though now usually a more easily maintained arrangement is used such as a square tapered or splined interface. These cotters have a short threaded section at the narrower end of the taper, which is used to hold the cotter in place with a washer and nut.
Because bicycle cotters were normally retained by a nut and washer, the distinction was unimportant in this context.