melena vs faecal matter

melena and faecal matter both are nouns.

Word	Noun	Adjective	Verb	Adverb
melena	Yes	No	No	No
faecal matter	Yes	No	No	No

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

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

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

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