This instrument, as its name (from the Greek, fire, up, a measure) indicates, is intended to measure the heat equivalent of radiation received from or going out toward the complete hemisphere above the plane of the measuring surface.
C. G. Abbot and L. B. Aldrich
The Pyranometer: An Instrument for Measuring Sky Radiation, 1916
Global horizontal irradiance (GHI) is the total solar flux available from the hemispheric sky dome that is incident on a horizontal surface. By convention global irradiance implies a hemispheric, 2n steradian, field of view on any surface. GHI is the sum of the solar radiation coming directly from the solar disk, also called the ...