The ratio of (1) the total output noise power within an output frequency band when the noise temperature of all input terminations is at the reference noise temperature, T0, at all frequencies that contribute to the output noise to (2) that part of (1) caused by the noise of the signal-input termination within the signal-input frequency band. (Ref. IEC 747‑1.)
NOTE 1 The abbreviation is often used in place of symbol ; however, symbol is preferred.
NOTE 2 This ratio may be expressed logarithmically in decibels (dB).