ball bearing

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A friction-reducing bearing consisting essentially of a ring-shaped track containing freely revolving hard metal balls against which a rotating shaft or other part turns.
• noun A hard ball used in such a bearing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun In engineering, a bearing assembly which uses spherical bearing balls as the rolling elements
• noun In common usage, an individual spherical ball, usually made of steel or other hard material, suitable for use in a ball-based rolling element bearing

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun bearings containing small metal balls