Longman Dictionary of Contemporary English
Wiktionary
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
n. a ballplayer who is designated to bat in place of the pitcher
Wikipedia
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.