BM vs melena

BM and melena both are nouns.

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

As nouns, melena is a hyponym of BM; that is, melena is a word with a more specific, narrower meaning than BM:

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

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