kingpin

noun

1. 1

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

2. 2

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

3. 3

   The most important person in an undertaking or organization.

4. 4

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