linch ‧ pin BrE AmE , lynchpin /ˈlɪntʃˌpɪn/ noun
[ Date: 1300-1400 ; Origin: linch 'pin fastening a wheel to an axle' (11-16 centuries) (from Old English lynis ) + pin ]
the linchpin of something the person or thing in a group, system etc that is most important, because everything depends on them