linchpin /linchˈ or linshˈpin/

1. A pin used to keep a wheel on its axle
2. A person or thing essential to a plan, organization, etc (figurative)

• backboards
• back boiler
• back boilers
• backbond
• backbone
• backboned
• backboneless
• backbones
• backbreaker
• backbreakers
• backbreaking
• back burner
• back-burner
• back burners
• back-calculate
• back-calculated
• back-calculates
• back-calculating
• back-calculation
• backcast
• backcasting
• backcasts
• back catalogue
• back catalogues
• back-chain