coincidence vs happenstance

coincidence and happenstance both are nouns.

Word	Noun	Adjective	Verb	Adverb
coincidence	Yes	No	No	No
happenstance	Yes	No	No	No

As nouns, coincidence and happenstance are synonyms defined as:

coincidence and happenstance: an event that might have been arranged although it was really accidental

coincidence (noun)	happenstance (noun)
the temporal property of two things happening at the same time	an event that might have been arranged although it was really accidental
the quality of occupying the same position or area in space
an event that might have been arranged although it was really accidental