Kingpin

noun

1. (motorized vehicles) The axis around which steered wheels pivot; a bolt that holds the axis in place—a kingbolt.
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. The most important person in an undertaking or organization.
4. The bolt holding together the truck of a roller skate or skateboard.