rest period vs breather

rest period and breather both are nouns.

Word	Noun	Adjective	Verb	Adverb
rest period	Yes	No	No	No
breather	Yes	No	No	No

As nouns, breather is a hyponym of rest period; that is, breather is a word with a more specific, narrower meaning than rest period:

Other hyponyms of rest period include breath, breathing place, breathing space, breathing spell, breathing time.

rest period (noun)	breather (noun)
a pause for relaxation	air passage provided by a retractable device containing intake and exhaust pipes; permits a submarine to stay submerged for extended periods of time
	a short respite