lynchpin

/L IH1 N CH P AE1 N/
noun
  1. 1

    A pin inserted through holes at the end of an axle or shaft, so as to secure a wheel or shaft-mounted device.

  2. 2

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

Source: https://en.wiktionary.org/wiki/linchpin, https://en.wiktionary.org/wiki/lynchpin