Loosely, the amount of thermal noise present in a system,
expressed in Kelvins.
Technically, it is the temperature of a passive system having an
available noise power per unit bandwidth equal to that of the actual
terminals. The noise temperature of a simple resistor is the actual
temperature of the resistor, while the noise temperature of another
component could be many times the observed absolute temperature.
Used in microwave communications and sometimes radar, it is the
equivalent of noise figure expressed in Kelvins
(Te = 290 [log-1 (NF dB/10) -1]).