cardiologist vs specialist

cardiologist and specialist both are nouns.

Word	Noun	Adjective	Verb	Adverb
cardiologist	Yes	No	No	No
specialist	Yes	No	No	No

As nouns, specialist is a hypernym of cardiologist; that is, specialist is a word with a broader meaning than cardiologist:

cardiologist: a specialist in cardiology; a specialist in the structure and function and disorders of the heart
specialist: practices one branch of medicine

Other hypernyms of cardiologist include medical specialist.

cardiologist (noun)	specialist (noun)
a specialist in cardiology; a specialist in the structure and function and disorders of the heart	an expert who is devoted to one occupation or branch of learning
	practices one branch of medicine