In the United States, cotter pin (also known as a cotter key or a split pin) is a metal fastener with two tines that are bent during installation, similar to a staple or rivet. Typically made of wire with a half-circular cross section, cotter pins come in multiple sizes and types.

In the UK, the term split pin has traditionally been used to describe the same device. The term "cotter pin" is reserved for the tapered round cotter that was used to fix bicycle pedal cranks to their shaft, being installed offset from the diameter of the driven crank, flat on one side to prevent rotation and tapered so as to tighten the entire union. The different usage between the two countries is often a cause of confusion when companies of both countries work together.