The brightness temperature (TB) of an electromagnetic radiation source is the temperature of a black body that would produce an observed spectral radiance, a value which is wavelength-dependent. For a given wavelength, specific intensity of black-body spectrum increases with temperature: even within the portion of the black-body spectrum to the longer-wavelength side of its peak, the energy per wavelength-interval grows with temperature. Brightness temperature is commonly used in radio astronomy where it can be usefully computed using the Rayleigh-Jeans law rather than the more complex Planck function. To base it on an actual intensity requires an extended source.
The term is used whether or not the EMR is thermal radiation, without necessarily having any connection to the actual temperature, often only as a means of describing/comparing sources. It is often cited for a prominent emission line such as that of a maser's wavelength.