The addition of the gradient of some function of space and time to the magnetic vector potential, and the addition of the negative of the partial derivative of the same function with respect to time, divided by the speed of light, to the electric scalar potential; this procedure gives different potentials but leaves the electric and magnetic fields unchanged.

An alteration of the phase of the fields of a gauge theory as a function of space and time which does not alter the value of any measurable physical quantity.