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)

• illustrates
• illustrating
• illustration
• illustrational
• illustrations
• illustrative
• illustratively
• illustrator
• illustrators
• illustratory
• illustrious
• illustriously
• illustriousness
• illustriousnesses
• illustrissimo
• illuvia
• illuvial
• illuviation
• illuviations
• illuvium
• illuviums
• ill-versed
• ill-will
• ill-wisher
• ill-wishers