Frequency deviation (Δf) is used in FM radio to describe the maximum instantaneous difference between an FM modulated frequency and the nominal carrier frequency. The term is sometimes mistakenly used as synonymous with frequency drift, which is an unintended offset of an oscillator from its nominal frequency.
The frequency deviation of a radio is of particular importance in relation to bandwidth, because less deviation means that more channels can fit into the same amount of frequency spectrum. The FM broadcasting range (87.5–108 MHz, NOTE: In some countries the 87.5–88.0 MHz part of the band is not used) uses a channel spacing of 200 kHz, with a maximum frequency deviation of 75 kHz, leaving a 25 kHz buffer above the highest and below the lowest frequency to reduce interaction with other channels. AM broadcasting uses a channel spacing of 10 kHz, but with amplitude modulation frequency deviation is irrelevant.
FM applications use peak deviations of 75 kHz (200 kHz spacing), 5 kHz (25 kHz spacing), 2.5 kHz (12.5 kHz spacing), and 2 kHz (8.33 kHz spacing).