In sports, the ball is a spherical (or ellipsoid, as in rugby and American football) shape that is used in some fashion for play. They can range in size from a half inch or so for marbles, to beach balls several feet in diameter. Materials also vary, again from the hard glass used in marbles, through many that are created out of natural materials like leather with an inflatable bladder, to simple hollow plastic spheres or molded foam.
In baseball, it is a pitch which does not enter the strike zone and is not struck at by the batter.
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