(noun) a locking pin inserted crosswise (as through the end of an axle or shaft) / (noun) one that serves to hold together parts or elements that exist or function as a unit
The Bretton Woods system, already set out in 1944, posited the dollar as the sole linchpin holding together the edifice of global trade and finance.
The linchpin of this social order was the Shah dynasty.
Missouri, the country's cartographic linchpin