cannonball vs cannon ball

cannonball and cannon ball both are nouns.

Word	Noun	Adjective	Verb	Adverb
cannonball	Yes	No	No	No
cannon ball	Yes	No	No	No

As nouns, cannonball and cannon ball are synonyms defined as:

cannonball and cannon ball: a solid projectile that in former times was fired from a cannon

Other synonyms of cannonball include round shot.

cannonball (noun)	cannon ball (noun)
a solid projectile that in former times was fired from a cannon	a solid projectile that in former times was fired from a cannon