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Longman Dictionary of Contemporary English
designated hitter
noun
EXAMPLES FROM CORPUS
▪ As with the designated hitter, there would be different rules in different leagues.
▪ Getting comfortable in his new role, he played six positions and was a designated hitter in 104 games.
▪ I was a designated hitter there, and every day I got three or four ABs.
▪ If flag art goes, then so does the designated hitter.
▪ That would allow the club to use Reggie Jefferson as a designated hitter.
▪ The Sox want Canseco, the designated hitter in 1995, to play right.
Wiktionary
designated hitter

n. 1 (context baseball English) A batter who is permitted to substitute for a pitcher in the batting rotation in Major League Baseball's American League. 2 (context colloquial English) A person who is asked to substitute for another in handling a portion of a task for which the other is less capable.

WordNet
designated hitter

n. a ballplayer who is designated to bat in place of the pitcher

Wikipedia
Designated hitter

In baseball, the designated hitter rule is the common name for Major League Baseball Rule 5.11, adopted by the American League in 1973. The rule allows teams to have one player, known as the designated hitter (abbreviated DH), to bat in place of the pitcher. Since 1973, most collegiate, amateur, and professional leagues have adopted the rule or some variant. MLB's National League and Nippon Professional Baseball's Central League are the most prominent professional leagues that do not use a designated hitter.

Usage examples of "designated hitter".

Actually, he was whacked out on the sidewalk, but re-entered the restaurant through the plate glass window, having been lifted off his feet by a shotgun blast fired by some other goombah's designated hitter.

Paul cops were clustered along the barrier, with two Minneapolis cops, including an Iowa kid who'd become the department's designated hitter.