What is the X in X-mas?
1550s, from Middle French fusion, from Latin fusionem (nominative fusio) "an outpouring, effusion," noun of action from fusus, past participle of fundere "pour, melt" (see found (v.2)). In nuclear physics sense, first recorded 1947; in jazz sense, by 1972.
fusion fu·sion (fyōō'zhən)
The act or procedure of liquefying or melting by the application of heat.
The merging of different elements into a union, as of vertebrae.
The mechanism by which both eyes blend slightly different images from each eye into a single image.
The growing together of two or more teeth as a result of the abnormal union of their formative organs.
A nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy.