chair vs backrest

chair and backrest both are nouns.

Word	Noun	Adjective	Verb	Adverb
chair	Yes	No	Yes	No
backrest	Yes	No	No	No

chair (noun)	backrest (noun)
the position of professor	a support that you can lean against while sitting
a seat for one person, with a support for the back
an instrument of execution by electrocution; resembles an ordinary seat for one person
a particular seat in an orchestra
the officer who presides at the meetings of an organization

chair (verb)	backrest (verb)
preside over
act or preside as chair, as of an academic department in a university