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)

• cirrhopod
• Cirrhopoda
• cirrhoses
• cirrhosis
• cirrhotic
• cirri
• cirriform
• cirrigrade
• cirriped
• cirripede
• cirripedes
• Cirripedia
• cirripeds
• cirro-cumuli
• cirro-cumulus
• cirrose
• cirro-strati
• cirro-stratus
• cirrous
• cirrus
• cirsoid
• ciré
• cirés
• cis-
• CIS