You asked:

what does the word fusion mean

The word 'fusion' has several distinct uses: as a noun, 1. an occurrence that involves the production of a union; 2. the state of being combined into one body; 3. the merging of adjacent sounds or syllables or words; 4. a nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy; 5. the combining of images from the two eyes to form a single visual percept.