BABIP

From SoSH

Batting Average of Balls in Play is a stat that calculates a player's batting average on the balls he puts into play, thus excluding strikeouts and HRs that leave the park. The formula is as follows: BABIP = (H - HR)/(AB - HR - K). Similar to Eric Van's CBA but with the HR aspect included.

BABIP has two primary functions. The most important use of BABIP is as a tool to test the "luckiness" of a pitcher. While pitchers sometimes have small influences on their BABIP allowed, a larger sample size will generally revert to a BABIP around .300, so a pitcher with an abnormally low or high BABIP could be having a deceptively good or bad season that is not indicative of his true ability. The .300 BABIP standard only applies to starting pitchers - there are much more exceptions among relief pitchers and knuckleballers.

Flukes among players can also be identified using BABIP, but it's not quite the same tool because players have a much stronger influence on BABIP. A strong BABIP could be indicative of a good line-drive swing. However, a really high BABIP could still be a sign that a player has been hitting over his head.