marriage vs union

marriage and union both are nouns.

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

As nouns, marriage and union are synonyms defined as:

marriage and union: the state of being a married couple voluntarily joined for life (or until divorce)

Other synonyms of marriage include matrimony, spousal relationship, wedlock.

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

Other hypernyms of marriage include unification.

marriage (noun)	union (noun)
the act of marrying; the nuptial ceremony	the act of making or becoming a single unit
two people who are married to each other	the act of pairing a male and female for reproductive purposes
the state of being a married couple voluntarily joined for life (or until divorce)	a device on a national flag emblematic of the union of two or more sovereignties (typically in the upper inner corner)
a close and intimate union	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)

marriage (adjective)	union (adjective)
	of trade unions