MIG (Metal-arc Inert Gas) or MAG (Metal-arc Active Gas) welding, both referred to in AWS terminology as GMAW (Gas Metal Arc Welding), is a continuous wire arc welding process in which heat is generated by an arc striking between a fused wire and the workpiece. Consequently, the wire performs both the function of an electrode and the function of contributing material to the joint, as the passage of current causes it to melt, and it is continuously fed into the welding zone by means of a flashlight.

The protective atmosphere required to allow the arc to operate, preventing contamination of the bath by the air atmosphere, is provided by a gas tributary to the flashlight, which, depending on its composition, can be termed active or inert: Argon (Ar), Helium (He) and mixtures between the two gases are defined as inert (MIG welding), any addition in mixture of Carbon Dioxide (CO2), Oxygen (O2 ), Nitrogen (N2) and Hydrogen (H2) lead the gas to be considered active (MAG welding).