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

continuous wave
CW

    (CW) A term from early radio history
   for a transmitter using an electron tube (valve) oscillator
   to constantly add energy to a tuned circuit connected to an
   antenna.

   The term is used in contrast with the use of a spark gap to
   initiate a damped sinusoidal wave in a tuned circuit consisting
   of an inductor and capacitor.  The energy in this circuit
   constantly changes between the capacitor's electrostatic field
   and the inductor's magnetic field.  The energy is then coupled
   to the radiating antenna, loosely (so as not to dampen the wave
   too quickly).

   Some radio amateurs understand "CW" to mean transmission by
   means a single frequency signal which is either on or off
   (e.g. Morse code), as opposed to a carrier which varies
   continuously in amplitude, frequency or phase.  Some would
   even call the former "unmodulated" even though turning on and
   off is actually the most extreme form of amplitude modulation.

   (2009-11-24)