myocardium vs cardiac muscle

myocardium and cardiac muscle both are nouns.

Word	Noun	Adjective	Verb	Adverb
myocardium	Yes	No	No	No
cardiac muscle	Yes	No	No	No

As nouns, cardiac muscle is a hypernym of myocardium; that is, cardiac muscle is a word with a broader meaning than myocardium:

myocardium: the middle muscular layer of the heart wall
cardiac muscle: the muscle tissue of the heart; adapted to continued rhythmic contraction

Other hypernyms of myocardium include heart muscle.

myocardium (noun)	cardiac muscle (noun)
the middle muscular layer of the heart wall	the muscle tissue of the heart; adapted to continued rhythmic contraction