fountain pen vs pen

fountain pen and pen both are nouns.

Word	Noun	Adjective	Verb	Adverb
fountain pen	Yes	No	No	No
pen	Yes	No	Yes	No

As nouns, pen is a hypernym of fountain pen; that is, pen is a word with a broader meaning than fountain pen:

fountain pen: a pen that is supplied with ink from a reservoir in its barrel
pen: a writing implement with a point from which ink flows

fountain pen (noun)	pen (noun)
a pen that is supplied with ink from a reservoir in its barrel	female swan
	a writing implement with a point from which ink flows
	an enclosure for confining livestock
	a correctional institution for those convicted of major crimes
	a portable enclosure in which babies may be left to play

fountain pen (verb)	pen (verb)
	produce a literary work