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

phase-shift keying

    (PSK) A digital modulation scheme that
   conveys data by changing the phase of a carrier wave.  The
   data can either determine the absolute phase relative to the
   unmodulated carrier or reference signal ("coherent phase-shift
   keying", CPSK) or the change in phase ("differential
   phase-shift keying", DPSK).  The number of different phases
   used determines the amount of data that can be transmitted in
   each cycle.  Each cycle can be considered to constitute one
   "symbol", e.g. with two possible phases, each cycle carries
   one bit.  The more phases that are used, the less tollerant
   to noise the transmissions becomes.

   Alternatives to PSK are amplitude-shift keying (ASK) and
   frequency-shift keying (FSK).

   (2010-01-26)