ballpoint vs ballpen

ballpoint and ballpen both are nouns.

Word	Noun	Adjective	Verb	Adverb
ballpoint	Yes	No	No	No
ballpen	Yes	No	No	No

As nouns, ballpoint and ballpen are synonyms defined as:

ballpoint and ballpen: a pen that has a small metal ball as the point of transfer of ink to paper

Other synonyms of ballpoint include ballpoint pen, Biro.

ballpoint (noun)	ballpen (noun)
a pen that has a small metal ball as the point of transfer of ink to paper	a pen that has a small metal ball as the point of transfer of ink to paper