union vs fusion

union and fusion both are nouns.

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

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

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

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

union (noun)	fusion (noun)
the act of making or becoming a single unit	the act of fusing (or melting) together
the act of pairing a male and female for reproductive purposes	correction of an unstable part of the spine by joining two or more vertebrae; usually done surgically but sometimes done by traction or immobilization
a device on a national flag emblematic of the union of two or more sovereignties (typically in the upper inner corner)	the combining of images from the two eyes to form a single visual percept
the occurrence of a uniting of separate parts	an occurrence that involves the production of a union
an organization of employees formed to bargain with the employer	a nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy
a political unit formed from previously independent people or organizations	the merging of adjacent sounds or syllables or words
a set containing all and only the members of two or more given sets	the state of being combined into one body
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)	fusion (adjective)
of trade unions