melaena vs dejection

melaena and dejection both are nouns.

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

As nouns, dejection is a hypernym of melaena; that is, dejection is a word with a broader meaning than melaena:

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

Other hypernyms of melaena include BM, faecal matter, faeces, fecal matter, feces, ordure, stool.

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