Frequency-shift keying (FSK) is frequency modulation in which the modulating signal shifts the output frequency between predetermined values.

Note 1: Usually, the instantaneous frequency is shifted between two discrete values termed the "mark frequency" and "space frequency". This is a noncoherent form of FSK.

Note 2: Coherent forms of FSK exist in which there is no phase discontinuity in the output signal. Synonyms frequency-shift modulation, frequency-shift signaling.

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