Well, it's not my definition, but here's how ASA defines it:
Rule 1 (Play): An attempt by a defensive player to retire a batter-runner or runner.
Then, under the Look Back Rule, there is this note regarding actions by the pitcher: A fake throw is considered a play.
Somewhere between those two definitions, lies the area where umpire judgment has to kick in. I've seen it taught in some clinics that ANY action by the pitcher that might reasonably cause the runner to react could be regarded as "a play".
Given that the intent of this rule is to speed up the game by cutting out any "cat and mouse" dancing around on the bases, not to get cheap and easy "gotcha" outs for the defense, my personal judgment tends to tip toward the runner's favor if there are any moves by the pitcher that cause the runner to react. You really have to take all of the elements of the play into consideration and judge each instance on its own merit.
Raising the arm up into a throwing position while facing the runner is good enough for me to say the pitcher is "faking a throw".
Holding the arm out to the side, away from the body- probably not a play, unless there is some forward movement of the arm.
Holding the ball at the side, in close to the body, then turning in place to look at the runner isn't likely to be enough to consider it a play.
Holding the ball at the side and then taking several quick steps toward the runner, could be considered a play.
Calling outs under the Look Back Rule should be saved for obvious gross violations by the runner, where the runner's actions are aimed at confusing the defense, gaining an unfair advantage or causing a delay to the game.