happenstance vs chance event

happenstance and chance event both are nouns.

Word	Noun	Adjective	Verb	Adverb
happenstance	Yes	No	No	No
chance event	Yes	No	No	No

As nouns, chance event is a hypernym of happenstance; that is, chance event is a word with a broader meaning than happenstance:

happenstance: an event that might have been arranged although it was really accidental
chance event: anything that happens suddenly or by chance without an apparent cause

Other hypernyms of happenstance include accident, fortuity, stroke.

happenstance (noun)	chance event (noun)
an event that might have been arranged although it was really accidental	anything that happens suddenly or by chance without an apparent cause