union vs combining

union and combining both are nouns.

Word	Noun	Adjective	Verb	Adverb
union	Yes	Yes	No	No
combining	Yes	No	No	No

As nouns, combining is a hypernym of union; that is, combining is a word with a broader meaning than union:

Other hypernyms of union include combination, compounding.

As nouns, combining is a hyponym of union; that is, combining is a word with a more specific, narrower meaning than union:

Other hyponyms of union include amphimixis, fusion, merger, unification, combine.

union (noun)	combining (noun)
the act of making or becoming a single unit	the act of combining things to form a new whole
the act of pairing a male and female for reproductive purposes	an occurrence that results in things being united
a device on a national flag emblematic of the union of two or more sovereignties (typically in the upper inner corner)
the occurrence of a uniting of separate parts
an organization of employees formed to bargain with the employer
a political unit formed from previously independent people or organizations
a set containing all and only the members of two or more given sets
healing process involving the growing together of the edges of a wound or the growing together of broken bones
the state of being joined or united or linked
the state of being a married couple voluntarily joined for life (or until divorce)

union (adjective)	combining (adjective)
of trade unions