Definition of "kingpin"

(transport) The axis around which steered wheels pivot; a bolt that holds the axis in place—a kingbolt.  examples

(transport) The pivot or connector between a tractor and trailer in the fifth-wheel coupling of semi-trailer truck.  examples

(ten-pin bowling) The pin at the centre of the triangle of bowling pins (originally the tallest pin in kayles). Sometimes also the headpin at the apex.  examples

The most important person in an undertaking or organization.  quotations  examples

The bolt holding together the truck of a roller skate or skateboard.  examples