reunion vs uniting

reunion and uniting both are nouns.

Word	Noun	Adjective	Verb	Adverb
reunion	Yes	No	No	No
uniting	Yes	No	No	No

As nouns, uniting is a hypernym of reunion; that is, uniting is a word with a broader meaning than reunion:

Other hypernyms of reunion include conjugation, jointure, unification, union.

reunion (noun)	uniting (noun)
the act of coming together again	the act of making or becoming a single unit
a party of former associates who have come together again	the combination of two or more commercial companies