Definition: The ball you have in play, of course, even if you have illegally substituted one ball for another.
A ball is considered in play from the moment you make a stroke at it from the teeing ground until you hole out. The exceptions are when it is lost, out of bounds or lifted, or another ball has been substituted.
"Ball in play" is a term used frequently throughout the Rules of Golf, and there are a lot of penalties for doing things you aren't supposed to with a ball in play. So unless you are certain that you are allowed to lift a ball or otherwise influence a ball in play (other that making a stroke), don't mess with it.
