From The Collaborative International Dictionary of English v.0.48 [gcide]:
Reliever \Re*liev"er\ (-?r), n.
One who, or that which, relieves.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
reliever
n 1: someone who takes the place of another (as when things get
dangerous or difficult); "the star had a stand-in for
dangerous scenes"; "we need extra employees for summer
fill-ins" [syn: stand-in, substitute, relief,
reliever, backup, backup man, fill-in]
2: a person who reduces the intensity (e.g., of fears) and calms
and pacifies; "a reliever of anxiety"; "an allayer of fears"
[syn: reliever, allayer, comforter]
3: a pitcher who does not start the game [syn: reliever,
relief pitcher, fireman]