Definition of linchpin noun from the Oxford Advanced American Dictionary

 

linchpin

 noun
(also lynchpin)noun
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 our health center.
See the Oxford Advanced Learner's Dictionary entry: linchpin