The Free On-line Dictionary of Computing (30 December 2018):

Frequency Shift Keying
FSK

    (FSK) The use of frequency modulation to
   transmit digital data, i.e. two different carrier
   frequencies are used to represent zero and one.

   FSK was originally used to transmit teleprinter messages by
   radio (RTTY) but can be used for most other types of radio
   and land-line digital telegraphy.  More than two frequencies
   can be used to increase transmission rates.

   (1997-07-14)