fighter aircraft vs warplane

fighter aircraft and warplane both are nouns.

Word	Noun	Adjective	Verb	Adverb
fighter aircraft	Yes	No	No	No
warplane	Yes	No	No	No

As nouns, warplane is a hypernym of fighter aircraft; that is, warplane is a word with a broader meaning than fighter aircraft:

fighter aircraft: a high-speed military or naval airplane designed to destroy enemy aircraft in the air
warplane: an aircraft designed and used for combat

Other hypernyms of fighter aircraft include aeroplane, airplane, plane, military plane.

fighter aircraft (noun)	warplane (noun)
a high-speed military or naval airplane designed to destroy enemy aircraft in the air	an aircraft designed and used for combat