A designated hitter, or DH, is a baseball player who is chosen at the start of a game to bat in place of the pitcher. If a DH is used, he will bat in his turn in the batting order but will not play a defensive (fielding) position. The pitcher will pitch and play defense, but will not bat in the batting order.
While he is in the role of the DH, the designated hitter may not play a field position, and he may only be replaced by another player not currently in the lineup. However, the designated hitter may change positions to become a regular position player at any point during the game. If he does so, his team forfeits the role of the designated hitter; thus the pitcher--or a substitute for the pitcher, such as a pinch hitter--must bat in the newly-opened spot in the order.
Originally, the rules of Major League Baseball stated that each player had to bat in his spot in the order. This meant that pitchers didn't get to bat every day like other players, as they only took the field every four or five days at most, and so were usually not very effective hitters. Babe Ruth was one notable exception; he began his career as a pitcher for the Boston Red Sox.
In 1973, in an effort to combat both declining attendance and declining offense, the American League adopted a rule stating that a team could designate a hitter to bat for the pitcher. On April 6, at Fenway Park, Ron Blomberg of the New York Yankees became the first designated hitter in major league baseball history. He was walked by Boston Red Sox pitcher Luis Tiant in his first plate appearance.
A team that uses the designated hitter rule has, in general, two options on how to use designated hitters. They can either rotate the DH spot among players in the lineup so as to give the players a bit of a rest without removing them from the lineup, or they can employ a full-time designated hitter. Players in the latter category include Edgar Martinez of the Seattle Mariners, who had a long career as a DH despite problems with his legs and weaker defensive numbers. This rule also changed the game in several ways; pitchers went deeper into games because they didn't need to be lifted for a pinch hitter, the double switch became mostly unnecessary, and older players whose careers and skills were on the wane had a chance to play for an extra year or two. George Brett, Carl Yastrzemski, and Paul Molitor were all able to extend their careers as designated hitters.
The DH and baseball culture
There is a great deal of controversy about whether the designated hitter rule should be continued. Purists complain that it breaks up the symmetry of the game, whereas advocates like the increased offensive output. Whether one is in favor of the designated hitter rule often depends on whether their favorite team is in the American League (which uses the designated hitter) or the National League (which does not).
The arguments for and against the designated hitter rule extend beyond the theory of what true baseball is. There are more practical reasons argued. Since American League pitchers do not have to bat, they may be more likely to intimidate opposing batters with brush-back pitches and beanballs, knowing that they will not not have to enter the batter's box and deal with possible retaliation from the opposing pitcher. However, pitching statistics tend to be better in the National League, as each team's pitcher is often a weak hitter with a poor batting average.
As of September 2005, there is a debate raging as to whether David Ortiz, regular DH for the Boston Red Sox deserves the Most Valuable Player award. Ortiz has had tremendous 2005 season at the plate, and proponents claim that he may actually be facing a handicap at the plate - it is harder to stay loose and warmed up when sitting on the bench for roughly 8/9ths of the game - and as such, is even further deserving of the honor. However, critics claim that a DH's "value" as a player is seriously degraded by the fact that they do not contribute in the field.
When games are played between American League and National League teams during regular season interleague play, the All-Star Game, or in the World Series, the home team's league rules are followed. Thus, when a National League team plays in American League ballparks, the team receives the benefit of a stronger hitter in its lineup instead of the pitcher. Conversely, when American League teams play in National League ballparks, they suffer a disadvantage, as their pitchers do not bat very often during the regular season. (From 1976-1985, the designated hitter rule was used in all World Series games played even-numbered years, with pitchers batting in odd numbered years.)
Commissioner Bud Selig has proposed that the road team's rules would be followed for Interleague games.
The designated hitter in amateur baseball
The use of the DH in amateur baseball is mixed. The primary difference between the DH in professional and amateur baseball is that the DH may bat in place of one player in any position in most amateur baseball leagues such as those that use NFHS rules. Most high school coaches tend to either "play nine" (use no DH) or use a DH in place of the weakest hitter in the lineup. This is likely due to the fact that in amateur baseball, pitchers play other positions when they're not pitching and therefore are (relatively) better hitters than professional pitchers, who receive little to no hitting instruction (other than routine batting practice).