A pitcher's winning percentage is calculated by dividing his total number of wins by his total number of decisions (wins plus losses). Pitchers who get the win or the loss are known as the "pitchers of record" in a game, and winning percentage indicates how frequently a pitcher wins when he is the pitcher of record.
The significance of winning percentage for the purpose of player evaluation has decreased among statisticians and baseball fans in the past few decades. The thinking is that a pitcher on a team with a bad offense (or bullpen) will often lose, even during games in which he pitched well. A pitcher can go nine innings without allowing a run, but he won't get the win if his team can't score.
During the time when pitchers tossed complete games routinely, bullpens were rarely at fault for losses. But today's specialization of relief pitchers has led to starters pitching fewer innings, leaving their win-loss fate in the hands of the hurlers who enter the game after them -- and, of course, the offense, too.
But even with the recent devaluation of winning percentage, many fans, announcers and players still place a lot of importance on the statistic -- and understandably so.
Luck can certainly play a factor in a whether a pitcher wins a game, but during the course of the season, many of the game's best starters tend to appear on the winning-percentage leaderboard. The same usually isn't true of relief pitchers, whose winning percentage is far more a product of circumstance.
Watch: Zack Greinke earns a win, increasing his winning percentage.
In A Call
"win-loss percentage," "win-loss record" (which describes the two-number statistic that encompasses the pieces of winning percentage; for instance, if a pitcher is 3-2, his winning percentage is .600)