Dictionary.net

(CW) A term from early radio historyfor a transmitter using an electron tube (valve) oscillatorto constantly add energy to a tuned circuit connected to anantenna.The term is used in contrast with the use of a spark gap toinitiate a damped sinusoidal wave in a tuned circuitconsisting of an inductor and capacitor. The energy inthis circuit constantly changes between the capacitor'selectrostatic field and the inductor's magnetic field. Theenergy is then coupled to the radiating antenna, loosely (soas not to dampen the wave too quickly).Some radio amateurs understand "CW" to mean transmission bymeans a single frequency signal which is either on or off(e.g. Morse code), as opposed to a carrier which variescontinuously in amplitude, frequency or phase. Some wouldeven call the former "unmodulated" even though turning on andoff is actually the most extreme form of amplitude modulation.