spot noise figure; spot noise factor (F or NF)

The ratio of (1) the total output noise power per unit bandwidth (spectral density) at a single output frequency 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 at the signal-input frequency.

NOTE 1 The abbreviation NF is often used in place of symbol F; however, symbol F is preferred.

NOTE 2 These quantities may be expressed logarithmically in decibels (dB).

References: 

JESD77-B, 2/00
JESD99B, 5/07
RS-311-A, 11/81
RS-353, 4/68