volcanic crater vs maar

volcanic crater and maar both are nouns.

Word	Noun	Adjective	Verb	Adverb
volcanic crater	Yes	No	No	No
maar	Yes	No	No	No

As nouns, maar is a hyponym of volcanic crater; that is, maar is a word with a more specific, narrower meaning than volcanic crater:

volcanic crater: a bowl-shaped geological formation at the top of a volcano
maar: a flat-bottomed volcanic crater that was formed by an explosion; often filled with water

Other hyponyms of volcanic crater include caldera.

volcanic crater (noun)	maar (noun)
a bowl-shaped geological formation at the top of a volcano	a flat-bottomed volcanic crater that was formed by an explosion; often filled with water