Square Wave refers to the shape of the voltage cycle in a welding arc. In an AC wave, a typical sinusoidal or sine wave, changes the voltage from positive to negative gradually, although many times a second. This means the voltage will not be consistent and at times will actually go through zero. Square Wave, on the other hand, changes the voltage from positive to negative almost instantly, making for a much smoother and more stable welding arc. This is particularly important when welding aluminum and when considering aspects like cleaning and penetrating action.