Definition of linchpin noun from the Oxford Advanced Learner's Dictionary

 

linchpin

 noun
(also lynchpin)noun
BrE BrE//ˈlɪntʃpɪn//
 
; NAmE NAmE//ˈlɪntʃpɪn//
 
 
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a person or thing that is the most important part of an organization, a plan, etc., because everything else depends on them or it Nurses are the linchpin of the health service. Word Origin late Middle English: from Old English lynis (in the sense ‘linchpin’) + pin.
See the Oxford Advanced American Dictionary entry: linchpin