Bearing ball
A steel ball that is part of a ball bearing assembly. Most front hubs use 10 3/16" balls on each side; Most rear hubs use 9 1/4" balls on each side. Most bottom brackets use 11 1/4" balls on each side.
Bearing balls are made in batches, and within a batch the size is very carefully controlled. From one batch to another, however, there can be a considerable variation in actual size of balls of the same nominal size. For this reason, balls from different batches should never be mixed in the same bearing.
Bearing balls come in several quality grades, designated by numbers. Grade 25 is the best that is available for bicycle applications. Grades 100, 200, even 300 are commonly used in bicycles.
from sheldon browns site i havent a baldy notion myself