noun

• (transport) The axis around which steered wheels pivot; a bolt that holds the axis in place—a kingbolt.
• (transport) The pivot or connector between a tractor and trailer in the fifth-wheel coupling of semi-trailer truck.
• (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.
• The most important person in an undertaking or organization.
• The bolt holding together the truck of a roller skate or skateboard.