stool vs melena

stool and melena both are nouns.

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

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

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

Other hyponyms of stool include dog do, dog shit, dog turd, doggy do, crap, dirt, poop, shit, shite, turd, droppings, dung, muck, meconium, melaena.

stool (noun)	melena (noun)
a simple seat without a back or arms	abnormally dark tarry feces containing blood (usually from gastrointestinal bleeding)
a plumbing fixture for defecation and urination
(forestry) the stump of a tree that has been felled or headed for the production of saplings
solid excretory product evacuated from the bowels

stool (verb)	melena (verb)
have a bowel movement
grow shoots in the form of stools or tillers
react to a decoy, of wildfowl
lure with a stool, as of wild fowl