co-occurrence vs unison

co-occurrence and unison both are nouns.

Word	Noun	Adjective	Verb	Adverb
co-occurrence	Yes	No	No	No
unison	Yes	No	No	No

As nouns, unison is a hyponym of co-occurrence; that is, unison is a word with a more specific, narrower meaning than co-occurrence:

co-occurrence: the temporal property of two things happening at the same time
unison: occurring together or simultaneously

Other hyponyms of co-occurrence include concomitance, overlap, contemporaneity, contemporaneousness.

co-occurrence (noun)	unison (noun)
the temporal property of two things happening at the same time	(music) two or more sounds or tones at the same pitch or in octaves
an event or situation that happens at the same time as or in connection with another	occurring together or simultaneously
	corresponding exactly