reliever

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

• noun One that provides relief.
• noun Baseball A relief pitcher.

from The Century Dictionary.

• noun One who or that which relieves or gives relief.
• noun In gunnery, an iron ring fixed to a handle by means of a socket, which serves to disengage the searcher of a gun when one of its points is retained in a hole.
• noun A garment kept for being lent out.
• noun In mech.: A release; a device for releasing some part of a mechanism at a certain time or position.
• noun A device, such as a spring or elastic substance, for preventing shocks on a machine or part.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, relieves.

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

• noun baseball A relief pitcher.
• noun Someone who fills in for another.

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

• noun a pitcher who does not start the game
• noun someone who takes the place of another (as when things get dangerous or difficult)
• noun a person who reduces the intensity (e.g., of fears) and calms and pacifies