Melting is the process by which a substance changes from the solid phase to the liquid phase. Melting is also known as fusion, although this term has several meanings in science. Melting occurs when the internal energy of a solid increases, usually through the application of heat or pressure, such that the molecules become less ordered.
Example
In ice cube melting into liquid water is a familiar example of the process. Another common example is melting butter in a hot pan.