Linchpin, sometimes spelled lynchpin, literally refers to a locking pin inserted crosswise, as at the end of an axle or shaft. In figurative use, linchpin refers to a person or thing that serves to hold together parts or elements that exist or function as a unit; such a linchpin is often understood as the most important part of a complex situation or system.

// Investors are betting that the new product line will be the linchpin that secures the company's place in the very competitive market in the years and decades to come.

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“When people tell the story of my life, when I tell this story of my life, Trisha doesn’t get much space, but she is a linchpin. For me the linchpin is that tiny bit of aid that holds things together when they might otherwise fall apart that keeps you rolling down the road to where you were already going. It’s not the engine, it’s not the track. It’s invisible but in the moment essential help.” — Alice Randall, My Black Country: A Journey Through Country Music’s Black Past, Present, and Future, 2024

In his 1857 novel, Tom Brown’s School Days, Thomas Hughes describes the “cowardly” custom of “taking the linch-pins out of the farmers’ and bagmens’ gigs at the fairs.” The linchpin in question held the wheel on the carriage, and removing it made it likely that the wheel would come off as the vehicle moved. Such a pin was called a lynis in Old English; Middle English speakers added pin to form lynspin. By the early 20th century, English speakers were using linchpin for anything as critical to a complex situation as a linchpin is to a wagon, as when Winston Churchill, in 1930, wrote of Canada and the role it played in the relationship between Great Britain and the United States, that “no state, no country, no band of men can more truly be described as the linchpin of peace and world progress.”