designated hitter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A player designated at the start of a game to bat instead of the pitcher in the lineup.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun baseball A batter who is permitted to substitute for a pitcher in the batting rotation in Major League Baseball's American League.
• noun colloquial A person who is asked to substitute for another in handling a portion of a task for which the other is less capable.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a ballplayer who is designated to bat in place of the pitcher