Source : Free On-Line Dictionary of Computing

continuous wave
     
         (CW) A term from early radio history, when
        the {spark gap} method of transmission was replaced by
        vacuum-tube oscillators.  A spark gap initiates a ringing,
        damped sinusoidal wave in a tuned circuit consisting of an
        inductor and capacitor.  The energy in this circuit is
        constantly changing between the capacitor's electrostatic
        field and the inductor's magnetic field.  The energy is then
        coupled, loosely (so as not to dampen the wave too quickly),
        to the radiating antenna.
     
        In contrast, a vacuum-tube oscillator constantly adds energy
        to the tuned circuit, compensating for the amount coupled to
        the antenna, and the transmitted energy or "wave," is
        therefore "continuous".
     
        Many (especially radio amateurs) continue to understand "CW"
        to mean transmission by means a signal of a single frequency
        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 an extreme form of
        amplitude modulation.
     
        (1995-03-15)