(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.