house fly vs housefly

house fly and housefly both are nouns.

Word	Noun	Adjective	Verb	Adverb
house fly	Yes	No	No	No
housefly	Yes	No	No	No

As nouns, house fly and housefly are synonyms defined as:

house fly and housefly: common fly that frequents human habitations and spreads many diseases

Other synonyms of house fly include Musca domestica.

house fly (noun)	housefly (noun)
common fly that frequents human habitations and spreads many diseases	common fly that frequents human habitations and spreads many diseases