From Longman Dictionary of Contemporary Englishthe linchpin of somethingthe linchpin of somethingMAINthe person or thing in a group, system etc that is most important, because everything depends on them → linchpin
Examples from the Corpus
the linchpin of something• The modesty of the Arab woman is the linchpin of the whole political system.• My mother had always been the linchpin of our family.• Erme was the linchpin of the whole thing.• Apparently she accepted that she was the primary caretaker of her children and the linchpin of family life. 