Linchpin (noun)

A pin or bolt inserted crosswise through the end of an axle to keep a wheel in place and prevent it from coming off.

Origin:

The term 'linchpin' is derived from the middle english word 'lynchpin', which is a combination of 'lynch' meaning 'axletree' and 'pin' meaning 'pin'. the term has been in use since the 15th century.

Examples:

1. The linchpin was the key component that held the wheel securely in place on the axle.
2. The mechanic checked the tightness of the linchpins on all the wheels before driving the car.
3. Without the linchpin, the wheel would have fallen off during the long journey.
4. The farmer replaced the broken linchpin and continued on with the plowing.
5. The linchpin was carefully removed so that the wheel could be changed.