karate vs martial art

karate and martial art both are nouns.

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

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

karate: a traditional Japanese system of unarmed combat; sharp blows and kicks are given to pressure-sensitive points on the body of the opponent
martial art: any of several Oriental arts of weaponless self-defense; usually practiced as a sport

karate (noun)	martial art (noun)
a traditional Japanese system of unarmed combat; sharp blows and kicks are given to pressure-sensitive points on the body of the opponent	any of several Oriental arts of weaponless self-defense; usually practiced as a sport