CW

A continuous wave is an electromagnetic wave of constant amplitude and frequency, a pure carrier, and information is carried by turning the wave on and off. Morse code is often transmitted using CW (on-off keying).

The term originally referred to the absence of modulation during the time when the carrier is 'on'; while a spark gap transmitter would modulate the signal based on the spark frequency, the 'on' time in a CW signal contains carrier only as one pure and continuous single-frequency waveform.

See:
 * Modes
 * Morse code/Codes and Alphabets
 * What is the best way to learn Morse?