BM vs excrement

BM and excrement both are nouns.

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

As nouns, excrement is a hypernym of BM; that is, excrement is a word with a broader meaning than BM:

BM: solid excretory product evacuated from the bowels
excrement: waste matter (as urine or sweat but especially feces) discharged from the body

Other hypernyms of BM include body waste, excreta, excretion, excretory product.

BM (noun)	excrement (noun)
solid excretory product evacuated from the bowels	waste matter (as urine or sweat but especially feces) discharged from the body