A ball bearing is a common term referring to either a type of rolling-element bearing (this usage is most commonly used by engineers), or the individual ball used in a ball bearing (this usage is more common among laypersons). The remainder of this entry uses the term ball for the individual component and “ball bearing” or just “bearing” for the assembly.

The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit the loads through the balls. Usually one of the races is held fixed. As one of the bearing races rotates it causes the balls to rotate as well. Because the balls are rolling they have a much lower coefficient of friction than if two flat surfaces were rotating on each other.

Ball bearings tend to have lower load capacity for their size than other kinds of rolling-element bearings due to the smaller contact area between the balls and races. However, they can tolerate some misalignment of the inner and outer races.

Compared to other bearing types, the ball bearing is the least expensive, primarily because of the low cost of producing the balls used in the bearing.