The designated hitter -- or "DH" -- is a player who bats in place of the pitcher. The pitcher still handles his regular duties when his team is on defense, so the designated hitter does not play in the field.
The rule was adopted by the American League in 1973, while pitchers have continued to hit in games played at National League parks. That will change in 2020, with MLB instituting a universal DH for one season as part of its health and safety protocols during the COVID-19 pandemic. It's possible the NL could revert to its traditional rules in 2021.
Clubs utilize the DH position in several ways, with some employing a full-time DH and others using it as a means to provide one of their other regular players with a partial day of rest. The position can be beneficial to aging or injury-prone players who are no longer capable of playing defense on a regular basis but can still offer value at the plate. Additionally, if teams have two strong hitters who play the same defensive position, they can use the DH spot to keep both players in the lineup.
Because the designated hitter position does not provide a defensive component, the DH typically is expected to produce in above-average fashion on offense.