accident vs happening

accident and happening both are nouns.

Word	Noun	Adjective	Verb	Adverb
accident	Yes	No	No	No
happening	Yes	No	No	No

As nouns, happening is a hypernym of accident; that is, happening is a word with a broader meaning than accident:

accident: anything that happens suddenly or by chance without an apparent cause
happening: an event that happens

Other hypernyms of accident include natural event, occurrence, occurrent.

accident (noun)	happening (noun)
anything that happens suddenly or by chance without an apparent cause	an event that happens
an unfortunate mishap; especially one causing damage or injury