fen vs marshland

fen and marshland both are nouns.

Word	Noun	Adjective	Verb	Adverb
fen	Yes	No	No	No
marshland	Yes	No	No	No

As nouns, fen and marshland are synonyms defined as:

fen and marshland: low-lying wet land with grassy vegetation; usually is a transition zone between land and water

Other synonyms of fen include fenland, marsh.

fen (noun)	marshland (noun)
low-lying wet land with grassy vegetation; usually is a transition zone between land and water	low-lying wet land with grassy vegetation; usually is a transition zone between land and water
100 fen equal 1 yuan in China