Watt per square meter

Does not exist on Wikipeida

And it's not because it's missing an "s"

Watts per square meter does not exist.

It is the  SI unit for  the power of electromagnetic radiation per unit area (radiative flux) incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface.

So it a measuremnt of  the power either landing on or leaving a surface,