playpen vs pen

playpen and pen both are nouns.

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

As nouns, playpen and pen are synonyms defined as:

playpen and pen: a portable enclosure in which babies may be left to play

playpen (noun)	pen (noun)
a portable enclosure in which babies may be left to play	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

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