What is "balljoint"

Usage examples of "balljoint".

Between a sketch of a bird's wing and a careful drawing of a balljoint was a little doodle of something with spiked wheels and spinning blades.

He'd walk by on his way back from the Coke machine or the bathroom and knocked a boxful of balljoint wrench attach­ments that Arnie was using all over the floor in Amie's stall.