relief pitcher

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

• noun A pitcher who replaces another during a game.

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

• noun baseball A pitcher who enters the game after the starting pitcher or another relief pitcher is removed due to injury, ineffectiveness, ejection from the game or fatigue.

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

• noun a pitcher who does not start the game