Frequency shift

In telecommunication, the term frequency shift has the following meanings:

• Any change in frequency
• Any change in the frequency of a radio transmitter or oscillator. (Note: In the radio regime, frequency shift is also called rf shift.)
• Frequency-shift telegraphy[?]
• 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
• An intentional frequency change used for modulation purposes

Source: from Federal Standard 1037C and from MIL-STD-188