From WordNet (r) 2.0 (August 2003) [wn]:
backrest
n : a support that you can lean against while sitting; "the back
of the dental chair was adjustable" [syn: back]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
backrest \back"rest\ n.
1. a support that you can lean against while sitting.
Syn: back.
[WordNet 1.5]