dejection vs melena

dejection and melena both are nouns.

Word	Noun	Adjective	Verb	Adverb
dejection	Yes	No	No	No
melena	Yes	No	No	No

As nouns, melena is a hyponym of dejection; that is, melena is a word with a more specific, narrower meaning than dejection:

dejection: solid excretory product evacuated from the bowels
melena: abnormally dark tarry feces containing blood (usually from gastrointestinal bleeding)

Other hyponyms of dejection include dog do, dog shit, dog turd, doggy do, crap, dirt, poop, shit, shite, turd, droppings, dung, muck, meconium, melaena.

dejection (noun)	melena (noun)
a state of melancholy depression	abnormally dark tarry feces containing blood (usually from gastrointestinal bleeding)
solid excretory product evacuated from the bowels