How do you determine if a reaction is a fusion reaction?
"Fusion" means two things are joined into a new thing.
To be a "fusion" reaction, two different particles or things need to come together to form a unique new, single entity. The fusion of hydrogen atoms into helium atoms is one example.
In contrast, "fission" is the spontaneous splitting or decomposition of a single item into two or more smaller items. Uranium radioactivity is a common example of a fission reaction.