January 11th, 2023
The American League adopted the “designated hitter” rule, whereby an additional player is used to bat for the pitcher. Because the pitcher is still part of the team's nine defensive players, the designated hitter -- or "DH" -- does not take the field on defense. A team that chooses not to select a DH prior to a game is barred from using a DH for the rest of that game.
Source: m.mlb.com/glossary/rules/designated-hitter-rule