n. The act or operation of melting or rendering fluid by heat, without the aid of a solvent: as, the fusion of ice or of metals.n. The state of being melted or dissolved by heat; a state of fluidity or flowing in consequence of heat: as, metals in fusion.n. Hence The act of uniting or blending together, or the state of being united or blended, as if through melting; complete union, as of previously diverse elements or individuals.n. Specifically In politics, the coalition of two parties or factions.n. Abundance; plenty; profusion: same as foison.n. In modern psychology: A mode of intimate connection of elementary mental processes, such that the connected elements are difficult of analysis, and the resulting complex approximates the simplicity of impression characteristic of the element itself.n. The product of this connection; the blend or fused complex.n. The perception set up by the concurrence of a number of simple tonal stimuli. See fusion, 6.