In electromagnetism, the Lorenz gauge, Lorentz Condition or Lorenz gauge condition is a partial gauge fixing of the electromagnetic vector potential. The condition is that ??. This does not completely fix the gauge: one can still make a gauge transformation where is a harmonic scalar function (that is, a scalar function satisfying , the equation of a massless scalar field).
The Lorenz condition is used to eliminate the redundant spin-0 component in the (1/2,1/2) representation of the Lorentz group. It is equally used for massive spin-1 fields where the concept of gauge transformations does not apply at all.
The Lorenz condition is named after Ludvig Lorenz. It is a Lorentz invariant condition, and is frequently called the "Lorentz condition" because of confusion with Hendrik Lorentz, after whom Lorentz covariance is named. Wikipedia, Lorentz Condition