A continuous wave (CW) is an electromagnetic wave of constant amplitude and frequency. Continuous wave is also the name given to an early method of radio transmission, in which an unmodulated carrier wave is switched on and off. Unlike modulated modes, there is no information (in the communications sense) embedded within the signal; rather, information is carried in the rhythm and spacing with which the signal is sent. CW is thus is a form of on-off keying (OOK).
Continuous-wave radio was called radiotelegraphy because like the telegraph, it worked by means of a simple switch to transmit Morse code. However, instead of controlling the electricity in a cross-country wire, the switch controlled the power sent to a radio transmitter. This mode is still in common use by amateur radio operators due to its simplicity and reliability.
CW is the basis of the continuous-wave radar system, where a continuous wave is transmitted by one aerial while a second aerial receives the reflected radio energy. Also, continuous-wave lasers are lasers which operate in a non-pulsed mode.
In amateur radio parlance, the terms "CW" and "Morse code" are often used interchangeably, despite the distinctions between the two (Morse code may be sent using sound and light, for example).