One of the oldest and most universal tools to measure a hitter's success at the plate, batting average is determined by dividing a player's hits by his total at-bats for a number between zero (shown as .000) and one (1.000). In recent years, the league-wide batting average has typically hovered around .250.
While batting average is a useful tool for measuring a player's ability at the plate, it isn't all-encompassing. For instance, batting average doesn't take into account the number of times a batter reaches base via walks or hit-by-pitches. And it doesn't take into account hit type (with a double, triple or home run being more valuable than a single).
Batting average can also be applied in evaluating pitchers. In this case, it is called either "opponents' batting average" or "batting average against," and it is determined by dividing the number of hits against a given pitcher by the number of at-bats against him.
BAA is very common in evaluating pitchers -- especially when assessing opponent handed-ness splits. A pitcher cannot have an ERA against left-handed hitters because they are interspersed with righties in lineups. So when a pitcher's ability against hitters from each side of the plate is being compared, it is usually done by using either BAA or OPS-against.
Batting average was created as a measure to judge the success of a hitter. For one season, in 1887, walks counted as hits as well. But after that season, it was determined that batting average should take into account only a batter's hits -- and not any other methods he used to reach base.
In A Call
"average," as a verb: "he's batting" or "he's hitting"