kingpin

noun

1. Bowling.
   1. headpin.
   2. the pin at the center; the number five pin.
2. Informal. the person of chief importance in a corporation, movement, undertaking, etc.
3. Informal. the chief element of any system, plan, or the like.
4. a kingbolt.
5. either of the pins that are a part of the mechanism for turning the front wheels in some automotive steering systems.

noun

1. the most important person in an organization
2. the crucial or most important feature of a theory, argument, etc
3. Also called (Brit): swivel pin a pivot pin that provides a steering joint in a motor vehicle by securing the stub axle to the axle beam
4. tenpin bowling the front pin in the triangular arrangement of the ten pins
5. (in ninepins) the central pin in the diamond pattern of the nine pins