headrest vs head restraint

headrest and head restraint both are nouns.

Word	Noun	Adjective	Verb	Adverb
headrest	Yes	No	No	No
head restraint	Yes	No	No	No

As nouns, headrest and head restraint are synonyms defined as:

headrest and head restraint: a cushion attached to the top of the back of an automobile's seat to prevent whiplash

headrest (noun)	head restraint (noun)
a rest for the head	a cushion attached to the top of the back of an automobile's seat to prevent whiplash
a cushion attached to the top of the back of an automobile's seat to prevent whiplash