Expected Batting Average (xBA) is a Statcast metric that measures the likelihood that a batted ball will become a hit.
Each batted ball is assigned an xBA based on how often comparable balls -- in terms of exit velocity, launch angle and, on certain types of batted balls, Sprint Speed -- have become hits since Statcast was implemented Major League wide in 2015. (As of January 2019, xBA now factors in a batter's seasonal Sprint Speed on "topped" or weakly hit" balls).
For example, a line drive to the outfield with an xBA of .700 is given that figure because balls with a similar exit velocity and launch angle have become hits seven out of 10 times.
Knowing the expected outcomes of each individual batted ball from a particular player over the course of a season allows for the formation of said player's Expected Batting Average on balls in play. Real-world strikeout totals are then added in, resulting in a player's seasonal Expected Batting Average based on the quality of contact, instead of the actual outcomes. Likewise, this exercise can be done for pitchers to get their Expected Batting Average against.
Expected Batting Average is derived from Hit Probability, a depreciated metric that was used in 2017-18 and also measured the likelihood that a batted ball would become a hit. The difference is that Hit Probability was represented as a percentage, while xBA presents numbers on a batting-average scale.
Beginning with 2019, the name Hit Probability has been retired in favor of Expected Batting Average.
Why it's useful
Expected Batting Average is more indicative of a player's skill than regular batting average, as xBA removes defense from the equation. Hitters, and likewise pitchers, are able to influence exit velocity and launch angle but have no control over what happens to a batted ball once it is put into play.