linchpin

A locking pin inserted in the end of a shaft, as in an axle, to prevent a wheel from slipping off.

A central cohesive element.

A pin inserted in the spindle of the axle of a vehicle to prevent the wheel from slipping off. Also axle-pin.

A pin used to prevent the wheel of a vehicle from sliding off the axletree.

a pin inserted through holes at the end of an axle, so as to secure a wheel

a central cohesive source of stability and security; a person or thing that is critical to a system or organisation.

pin inserted through an axletree to hold a wheel on

a central cohesive source of support and stability