melena vs dejection

melena and dejection both are nouns.

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

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

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

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

melena (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