From Glossary of Meteorology
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(Or radiant flux density.) A radiometric term for the rate at which radiant energy in a radiation field is transferred across a unit area of a surface (real or imaginary) in a hemisphere of directions.

In general, irradiance depends on the orientation of the surface. The radiant energy may be confined to a narrow range of frequencies (spectral or monochromatic irradiance) or integrated over a broad range of frequencies. Irradiance follows from radiance but not, in general, vice versa. The photometric equivalent of irradiance is illuminance, obtained by integrating spectral irradiance times luminous efficiency over the visible spectrum.
See Poynting vector.