hair ball vs hairball

hair ball and hairball both are nouns.

Word	Noun	Adjective	Verb	Adverb
hair ball	Yes	No	No	No
hairball	Yes	No	No	No

As nouns, hair ball and hairball are synonyms defined as:

hair ball and hairball: a compact mass of hair that forms in the alimentary canal (especially in the stomach of animals as a result of licking fur)

Other synonyms of hair ball include trichobezoar.

hair ball (noun)	hairball (noun)
a compact mass of hair that forms in the alimentary canal (especially in the stomach of animals as a result of licking fur)	a compact mass of hair that forms in the alimentary canal (especially in the stomach of animals as a result of licking fur)