Any change in frequency. 2. Any change in the frequency of a radio transmitter or oscillator. Note: In the radio regime, frequency shift is also called rf shift. 3. See frequency-shift telegraphy. 4. In facsimile, a frequency modulation system where one frequency represents picture black and another frequency represents picture white. Frequencies between these two limits may represent shades of gray. 5. An intentional frequency change used for modulation purposes.