combining vs union

combining and union both are nouns.

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

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

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

combining (noun)	union (noun)
the act of combining things to form a new whole	the act of making or becoming a single unit
an occurrence that results in things being united	the act of pairing a male and female for reproductive purposes
	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)

combining (adjective)	union (adjective)
	of trade unions