In baseball, batting average (BA) is determined by dividing a player's hits by their total at-bats. It is usually rounded to three decimal places and read without the decimal: A player with a batting average of .300 is said to be "batting three hundred". If necessary to break ties, batting averages could be taken beyond the .001 measurement. In this context, .001 is considered a "point", such that a .235 batter is five points higher than a .230 batter.

Henry Chadwick, an English statistician raised on cricket, was an influential figure in the early history of baseball. He is credited with creating the modern box score, in 1859, and the practice of denoting a strikeout with a "K". Chadwick wrote in 1869: "In making up a score at the close of the match the record should be as follows:–Name of player, total number of times the first base was made by clean hits, total bases so made, left on bases after clean hits, and the number of times the first base has been made on errors..." This led to the recording of "clean" hits—times a batter reached base without benefit of an error. In 1869, another early baseball proponent, Alfred Wright, published an end-of-season summary that included the average number of times a batter had "clean" hits on a per game basis. In 1871, a writer for the New York Clipper, Hervie Dobson, proposed that a batter's "average is found by dividing his total 'times first base on clean hits' by his total number of times he went to the bat"—hits divided by at bats. By 1874, some teams were calculating batting averages, and by 1876, it was being calculated by all teams of the National League.

In modern times, a season batting average of .300 or higher is considered to be excellent, and an average higher than .400 is a nearly unachievable goal. The last Major League Baseball (MLB) player to do so, with enough plate appearances to qualify for the batting championship, was Ted Williams of the Boston Red Sox, who hit .406 in 1941.

Note that batting averages are rounded; entering the final day of the 1941 season, Williams was at 179-for-448, which is .39955 and would have been recorded as .400 via rounding. However, Williams played in both games of a doubleheader, went 6-for-8, and ended the season 185-for-456, which is .40570 and becomes .406 when rounded.

Since 1941, no American League or National League player has hit .400 or above—the highest single-season average in those leagues has been .394 by Tony Gwynn of the San Diego Padres in 1994. Wade Boggs hit .401 over a 162-game span with Boston from June 9, 1985, to June 6, 1986, but never hit above .368 for an MLB season. There have been numerous attempts to explain the disappearance of the .400 hitter, with one of the more rigorous discussions of this question appearing in Stephen Jay Gould's 1996 book Full House. Josh Gibson hit .466 in 1943 while playing for the Homestead Grays of the Negro National League, one of several leagues within Negro league baseball that are now recognized by MLB.

Gibson holds the record for highest major-league career batting average at .371, four points higher than Ty Cobb, who has the second-highest career average at .367. The record for lowest career batting average for a player with more than 2,500 at-bats belongs to Bill Bergen, a catcher who played from 1901 to 1911 and recorded a .170 average in 3,028 career at-bats.

Hugh Duffy, who played from 1888 to 1906, is credited with the highest single-season batting average, having hit .440 in 1894. The modern-era (post-1900) record for highest batting average for a season is held by Nap Lajoie, who hit .426 in 1901, the first year of play for the American League. The modern-era record for the lowest batting average for a player that qualified for the batting title is held by Chris Davis, who hit .168 in 2018. While finishing six plate appearances short of qualifying for the batting title, Adam Dunn of the Chicago White Sox hit .159 for the 2011 season, nine points lower than the record. The highest batting average for a rookie was .408 in 1911 by Shoeless Joe Jackson.

The league batting average in MLB for the 2018 season was .248, with the highest modern-era MLB average being .296 in 1930, and the lowest being .237 in 1968. For non-pitchers, a batting average below .230 is often considered poor, and one below .200 is usually unacceptable. This latter level is sometimes referred to as "The Mendoza Line", named for Mario Mendoza—a lifetime .215 hitter but a good defensive shortstop.