breath vs expiration

breath and expiration both are nouns.

Word	Noun	Adjective	Verb	Adverb
breath	Yes	No	No	No
expiration	Yes	No	No	No

As nouns, expiration is a hyponym of breath; that is, expiration is a word with a more specific, narrower meaning than breath:

Other hyponyms of breath include breathing out, exhalation, aspiration, breathing in, inhalation, inspiration, intake.

breath (noun)	expiration (noun)
the process of taking in and expelling air during breathing	the act of expelling air from the lungs
an indirect suggestion	euphemistic expressions for death
a slight movement of the air	a coming to an end of a contract period
the air that is inhaled and exhaled in respiration
a short respite