[fu.sion] n, often attrib [L fusion-, fusio, fr. fundere] (1555) 1: the act or process of liquefying or rendering plastic by heat
2: a union by or as if by melting: as a: a merging of diverse, distinct, or separate elements into a unified whole b: a political partnership: coalition c: popular music combining different styles (as jazz and rock)
3: the union of atomic nuclei to form heavier nuclei resulting in the release of enormous quantities of energy when certain light elements unite