Source : Free On-Line Dictionary of Computing

Frequency Shift Keying
     
         (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)