An inning pitched (or IP) is a statistic used to measure playing time for pitchers. It is a measure of how many innings a pitcher has played based on three opposing players (batters of runners) put out per inning. One out counts as a third of an inning, ⅓, but it is usually written in baseball statistics as a decimal, 0.1.
Innings pitched is not a reliable measure of how many batters or runners a pitcher dealt with, or how many pitched were thrown. It is possible for a pitcher to enter a game and allow several hits or batters to reach a base on balls, by way of a hit batsman etc., or even give up runs and then be pulled out of a game without any batter or runner being called out.
However, normally a pitcher who starts a game will earn a higher number of innings pitched, because normally they would be kept in at least long enough to earn a win decision, which required 5.0 innings pitched. Relief pitchers can vary in the number of innings pitched per game.
Innings pitched is often used to gauge a pitcher's tendency to strike batters out or walk them. It is directly used in calculating a pitcher's earned runs average or "ERA". Similar stats such as "K/9" and "BB/9" assess a pitcher's ability to strike batters out or walk them over an average game of 9 innings pitched.