Innings pitched measures the number of innings a pitcher remains in a game. Because there are three outs in an inning, each out recorded represents one-third of an inning pitched.
Just because a pitcher appears in a game, doesn't mean he will record an inning pitched or even a third of an inning pitched. In order for a pitcher's IP total to increase, he must be pitching while an out is recorded. (This includes pickoffs and caught stealings, while double plays are worth two-thirds of an inning pitched.) Pitchers are not credited for a third of an inning pitched if a batter reaches on an error because an out was never recorded.
In A Call
"innings," "innings of work," "frames"