In physics, melting is the process of heating a solid substance to a point (called melting point) where it turns liquid. An object, which has melted, is molten. Freezing is the opposite of melting. It is the process of turning a liquid to a solid. Consequently the freezing point is the same temperature as the melting point. See the phase article for more detail.
Not only the heating to the melting point, but also the melting itself requires heat, see latent heat.
In genetics, melting DNA means to separate the double-stranded DNA into two single strands by heating or the use of chemicals.
See also : Polymerase chain reaction