From The Collaborative International Dictionary of English v.0.48 :
backrest \back"rest\ n.
1. a support that you can lean against while sitting.
Syn: back.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) :
backrest
n 1: a support that you can lean against while sitting; "the
back of the dental chair was adjustable" [syn: back,
backrest]