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