1. For plane-wave radiation incident on a scattering object or a scattering medium, the ratio of the intensity scattered in the direction toward the source to the incident irradiance. So defined, the backscattering cross section has units of area per unit solid angle, for example, square meters per steradian. 2. In common usage, synonymous with radar cross section, although this can be confusing because the radar cross section is 4π times the backscattering cross section as defined in 1) and has units of area, for example, square meters. See differential backscattering cross section; compare radar cross section.