noun (n)

• a central cohesive source of support and stability(noun.cognition)

  Example:
  He is the linchpin of this firm.
  

  Synonym:
  anchor, backbone, keystone, lynchpin, mainstay
  

  source: wordnet30
• pin inserted through an axletree to hold a wheel on(noun.artifact)

  Synonym:
  lynchpin
  

  source: wordnet30
• A pin used to prevent the wheel of a vehicle from sliding off the axletree.(noun)

  source: webster1913