union vs marriage

union and marriage both are nouns.

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

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

As nouns, union and marriage are synonyms defined as:

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

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

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