"Fusion" comes from the Latinfundere "to pour or melt." The eventual meaning "melting by heat" occurs by the 1550s, "blending" by 1776, and the atomic reference is from 1947. Nuclear fusion is how stars shine (via collisions of atoms so that they fuse into different atoms, releasing heat). For fusion to be sustainable, it must be able to run by itself in a more-or-less controlled fashion, just as a star does, or perhaps a small child. Links.NightCafe Image Source