The act of joining two Metallic surfaces together by Melting a metal or alloy (referred to as solder) with a lower melting point into the space between them. This can be done by heating both the solder and the surfaces using a Soldering Iron, a heat gun, or a flame. Differs from Welding in that the metals to be joined are themselves not heated all the way to melting point.
When higher melting point solder is used, it is referred to as "brazing".