Batting average on balls in play

From BR Bullpen

Batting average on balls in play is a measure of the number of batted balls that safely fall in for a hit not including home runs.

BABIP = (H-HR) / (AB-K-HR+SF)

where H is hits, HR is home runs, AB is at bats, K is strikeouts, and SF is sacrifice flies.

There are many factors that affect BABIP, including batted ball types, ballparks, team defense, foot speed, luck or randomness. As with any stat, sample size is also an important consideration here.