union vs coalition

union and coalition both are nouns.

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

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

union: the act of making or becoming a single unit
coalition: the union of diverse things into one body or form or group; the growing together of parts

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

union (noun)	coalition (noun)
the act of making or becoming a single unit	the union of diverse things into one body or form or group; the growing together of parts
the act of pairing a male and female for reproductive purposes	an organization of people (or countries) involved in a pact or treaty
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 combined into one body
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)	coalition (adjective)
of trade unions