melaena vs feces

melaena and feces both are nouns.

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

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

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

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

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