pocket knife vs pocketknife

pocket knife and pocketknife both are nouns.

Word	Noun	Adjective	Verb	Adverb
pocket knife	Yes	No	No	No
pocketknife	Yes	No	No	No

As nouns, pocket knife and pocketknife are synonyms defined as:

pocket knife and pocketknife: a knife with a blade that folds into the handle; suitable for carrying in the pocket

pocket knife (noun)	pocketknife (noun)
a knife with a blade that folds into the handle; suitable for carrying in the pocket	a knife with a blade that folds into the handle; suitable for carrying in the pocket