linch·pin or lynch·pin 
(lĭnchpĭn′)
(lĭnchpĭn′)n.
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: Reduced spending is the linchpin of their economic program.
[Middle English linspin : lins, linchpin (from Old English lynis) + pin, pin (from Old English pinn; see PIN).]