combining vs fusion

combining and fusion both are nouns.

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

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

combining (noun)	fusion (noun)
the act of combining things to form a new whole	the act of fusing (or melting) together
an occurrence that results in things being united	correction of an unstable part of the spine by joining two or more vertebrae; usually done surgically but sometimes done by traction or immobilization
	the combining of images from the two eyes to form a single visual percept
	an occurrence that involves the production of a union
	a nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy
	the merging of adjacent sounds or syllables or words
	the state of being combined into one body