A ball-peen hammer.
ball-peen hammer definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a hammer with one end of the head rounded and the other end flat
ball-peen hammer definition by American Heritage Dictionary
A hammer having one end of the head hemispherical and used in working metal.