A linchpin, also spelled linch pin, lynchpin, or lynch pin, is a fastener used to prevent a wheel or other rotating part from sliding off the axle it is riding on. The word is first attested in the 14th century and derives from Middle English elements meaning "axletree pin".[1]
Securing implements onto the three-point hitch of a tractor is an example of application. Linchpins may also be used in place of an R-clip for securing hitch pins.