expiration vs breathing out

expiration and breathing out both are nouns.

Word	Noun	Adjective	Verb	Adverb
expiration	Yes	No	No	No
breathing out	Yes	No	No	No

As nouns, expiration and breathing out are synonyms defined as:

Other synonyms of expiration include exhalation.

expiration (noun)	breathing out (noun)
the act of expelling air from the lungs	the act of expelling air from the lungs
euphemistic expressions for death
a coming to an end of a contract period