ninjitsu vs martial art

ninjitsu and martial art both are nouns.

Word	Noun	Adjective	Verb	Adverb
ninjitsu	Yes	No	No	No
martial art	Yes	No	No	No

As nouns, martial art is a hypernym of ninjitsu; that is, martial art is a word with a broader meaning than ninjitsu:

ninjitsu: the traditional Japanese method of espionage; involves stealthy movements and the use of camouflage
martial art: any of several Oriental arts of weaponless self-defense; usually practiced as a sport

ninjitsu (noun)	martial art (noun)
the traditional Japanese method of espionage; involves stealthy movements and the use of camouflage	any of several Oriental arts of weaponless self-defense; usually practiced as a sport