kingpin

noun

1. The axis around which steered wheels pivot; a bolt that holds the axis in place.

   “The employees brought back the apparently disappointing news that almost all components showed signs of wear. The sole exceptions were the kingpins, which remained virtually unworn. To the employees’ surprise, [Henry] Ford, instead of expressing pride in his well-made kingpins, declared that the kingpins were overbuilt, and that in the future they should be made more cheaply.”

2. The pivot or connector between a tractor and trailer in the fifth-wheel coupling of semi-trailer truck.
3. (figuratively, informal)The most important person in a usually evil undertaking or organization, often a druglord.

   “The rare salarian females are cloistered on their worlds out of tradition and respect. Powerful female Dalatrasses are dynasts and political kingpins. They determine the political course of their respective regions through shrewd negotiation.”

   “As anyone who has ever paid remote attention to global affairs might have predicted, violence has broken out across several Mexican states in the aftermath of the killing – which is generally what happens when you take out a cartel kingpin.”

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