Glossary of Terms:
 
FSK
Frequency Shift Keying — A method of converting digital information into a signal which can be transmitted and recorded using a conventional audio system. FSK is used by MIDI sequencers to generate timing (clock) information so that it can be recorded on a conventional audio tape recorder.
Source: Yamaha

Also refered as:
  Frequency Shift Keying
Related terms:
  MIDI   clock   sequencer