act of God vs calamity

act of God and calamity both are nouns.

Word	Noun	Adjective	Verb	Adverb
act of God	Yes	No	No	No
calamity	Yes	No	No	No

As nouns, calamity is a hypernym of act of God; that is, calamity is a word with a broader meaning than act of God:

act of God: a natural and unavoidable catastrophe that interrupts the expected course of events
calamity: an event resulting in great loss and misfortune

Other hypernyms of act of God include cataclysm, catastrophe, disaster, tragedy.

act of God (noun)	calamity (noun)
a natural and unavoidable catastrophe that interrupts the expected course of events	an event resulting in great loss and misfortune