Definitions for "Power density"
(1) Power in watts per hertz, or the total power in a band of frequencies divided by the bandwidth in hertz; and (2) in optoelectronics, a colloquial synonym for irradiance.
Radiant power incident on a small sphere, divided by the cross-sectional area of that sphere.
power supplied in a fuel cell by electrode surface unit (W/cm2)
The amount of power dissipated from a film resistor through the substrate measured in W-cm-2.