kingpin

Short definition from Wiktionary

Noun

1. 1
   The axis around which steered wheels pivot; a bolt that holds the axis in place—a kingbolt.
2. 2
   The pivot or connector between a tractor and trailer in the fifth-wheel coupling of semi-trailer truck.
3. 3
   The most important person in a usually evil undertaking or organization, often a druglord.
4. 4
   The pin at the centre of the triangle of bowling pins (originally the tallest pin in kayles). Sometimes also the headpin at the apex.
5. 5
   The bolt holding together the truck of a roller skate or skateboard.