hairball vs hair ball

hairball and hair ball both are nouns.

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

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

hairball and hair ball: 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 hairball include trichobezoar.

hairball (noun)	hair ball (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)