Definitions for "Brightness temperature"
A measure of the intensity of a region of the sky, based on the temperature of a black body that would emit radiation of the observed intensity at the given observing frequency. Frequently computed in radio astronomy using the Rayleigh-Jeans approximation, in which case it is directly proportional to the intensity per unit area. For thermal emission mechanisms, it may correspond to a physical temperature in an emitting region.
The temperature of a blackbody radiating the same amount of energy per unit area at the wavelengths under consideration as the observed body.
The temperature an object/surface appears to have when measuring the intensity of its emitted radiation at a particular frequency/wavelength.