linch pin, also spelled linch pin, lynchpin, or lynch pin, is a used to prevent a or other rotating part from sliding off the it is riding on. It may be named after , the English mechanic who first patented its use in 1727, but and dictionaries date its use to the 13th century and 14th century respectively, from the word "Lynspin".

Linchpins are commonly used in the agricultural industry for securing implements onto the three point hitch of a . Linchpins may also be used in place of an for securing hitch pins.