outbreak vs happening

outbreak and happening both are nouns.

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

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

outbreak: a sudden violent spontaneous occurrence (usually of some undesirable condition)
happening: an event that happens

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

outbreak (noun)	happening (noun)
a sudden violent spontaneous occurrence (usually of some undesirable condition)	an event that happens