hematologist vs specialist

hematologist and specialist both are nouns.

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

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

hematologist: a doctor who specializes in diseases of the blood and blood-forming organs
specialist: practices one branch of medicine

Other hypernyms of hematologist include medical specialist.

hematologist (noun)	specialist (noun)
a doctor who specializes in diseases of the blood and blood-forming organs	an expert who is devoted to one occupation or branch of learning
	practices one branch of medicine