melaena vs faeces

melaena and faeces both are nouns.

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

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

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

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

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