1. A radiometric term for the rate at which radiant energy is transferred across a unit area of a surface, commonly measured in watts per square meter.
2. 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 a surface. The radiant energy may be confined to a narrow range of frequencies (spectral or monochromatic irradiance which is characterized by a single frequency) or integrated over a broad range of frequencies.