linchpin
noun
[ ˈlɪn(t)ʃpɪn ]
• a person or thing vital to an enterprise or organization.
• "nurses are the linchpin of the National Health Service"
• a pin passed through the end of an axle to keep a wheel in position.
Origin:
late Middle English: from Old English lynis (in the sense ‘linchpin’) + pin.
