Free-space loss

In telecommunication, free-space loss is the signal attenuation that would result if all absorbing, diffracting, obstructing, refracting, scattering, and reflecting influences were sufficiently removed so as to have no effect on its propagation.

Note: Free-space loss is primarily caused by beam divergence, i.e. , signal energy spreading over larger areas at increased distances from the source.

Source: from Federal Standard 1037C and from MIL-STD-188