Cotter pin may refer to:
In U.S. usage: Split pin, a metal fastener with two tines that are bent during installation used to fasten metal together, like with a staple or rivet Hairpin cotter pin, more commonly known as an "R-clip" Circle cotter, a ring-shaped cotter pin
In British usage: Cotter (pin), in mechanical engineering a pin or wedge passing through a hole to fix parts tightly together