linchpin
1. A locking pin inserted in the end of a shaft, as in an axle, to prevent a wheel from slipping off.
2. A central cohesive element
3. a person or thing regarded as an essential or coordinating element
Usage
Like a linchpin, the little fastener used to prevent a wheel from sliding off the axle, staff and technical sergeants may not be well-known, but they are indispensable. They are the middle ranks in the enlisted rank structure, essential building blocks of the enlisted force and are often the Air Force’s technical experts.
dvidshub.net
Etymology
also linch-pin, late 14c., earlier linspin, from M.E. lins "axletree" (see linch) + pin (n.). The peg that holds a wheel on an axle; now mainly figurative.
