The maximum permissible departure by the center frequency of the frequency band occupied by an emission from the assigned frequency, or by the characteristic frequency of an emission from the reference frequency. Frequency tolerance is expressed in parts per 106 or in hertz. Note: In the United States, frequency tolerance is expressed in parts per 10n, in hertz, or in percentages. Frequency tolerance includes both the initial setting tolerance and excursions related to short- and long-term instability and aging. 2. The maximum permissible departure by the center frequency of the frequency band occupied by an emission from the assigned frequency or, by the characteristic feequency of an emission from the reference frequency. The frequency tolerance is expressed as a percentage or in Hertz.