From AMS Glossary
In an electromagnetic wave, the rate of power flow in a specific direction at a particular point in a transmission medium, expressed as energy per unit time (power, or radiant flux) per unit cross-sectional area normal to the direction of propagation.
The power density generally diminishes with increasing distance from the source as a result of absorption, reflection, scattering, and possibly other effects, as well as geometric spreading of the beam. For surfaces or objects that intercept the radiation at a sufficiently long distance from the source, the propagating energy may be regarded as plane-wave or parallel-beam radiation. Then the power density is the same as the irradiance at a surface normal to the beam.