A kingpin is a person or thing that is central or essential to an organization, operation, or system. It can refer to:

• A person who holds the most power or influence within a group (e.g., the leader or key decision-maker).
• The principal or most important component of a larger mechanism or plan.
• In bowling, the central front pin in the triangular formation, and by extension the headpin or a pivotal pin.

If you’d like, I can tailor the definition to a specific context (business, crime, sports, machinery, etc.).