linchpin

UK: /ˈlɪntʃpɪn/ US: /ˈlɪntʃˌpɪn/ ,(linch′pin′)

WordReference Random House Learner's Dictionary of American English © 2025

linch•pin /ˈlɪntʃˌpɪn/   n. [countable]

1. Mechanical Engineering a pin inserted through the end of part of an axle to keep a wheel on.
2. something that holds the various parts or elements of a complicated structure together:
   The linchpin of the campaign was the heavy use of advertising.

WordReference Random House Unabridged Dictionary of American English © 2025

linch•pin  (linch′pin′), n. 

1. Mechanical Engineering a pin inserted through the end of an axletree to keep the wheel on.
2. something that holds the various elements of a complicated structure together:
   The monarchy was the linchpin of the nation's traditions and society.

Also, lynchpin. 

• unexplained alteration of Middle English lynspin, equivalent. to lyns, Old English lynis axle-pin (cognate with German Lünse) + pin pin 1350–1400

Collins Concise English Dictionary © HarperCollins Publishers::

linchpin, lynchpin / ˈlɪntʃˌpɪn/ noun

1. a pin placed transversely through an axle to keep a wheel in position
2. a person or thing regarded as an essential or coordinating element: the linchpin of the company

Etymology: 14th Century lynspin, from Old English lynis