Morse codewave

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CW is a mode of transmission to the same extent as AM, FM, SSB, or other digital modes. The CW mode stands for Continuous Wave, which is the manipulation of a continuous AM carrier over the air waves to produce an intelligible audio signal* when mixed with a heterodyne signal from the BFO (Beat Frequency Oscillator) stage of a receiver.

The resulting code is similar to what is known as Morse code, which was the precursor of CW and strictly used over wires. The American and Continental codes were significantly different than that of CW.

CW uses the international code and is primarily used over the air waves. Though most commercial traffic has now ceased operation using that mode, amateur radio operators are still very much using CW, primarily for two reasons: At equal power, it carries much further than a voice signal and the bandwidth of CW is much narrower than that of an SSB or AM signal. In fact, you could easily have more than 15 different CW transmissions, in comparison to only one SSB voice transmission.

Older Non-Directional Beacons used in air navigation use CW to transmit their identifier, which requires ADF receivers to include a Beat frequency oscillator to make the morse audible. In this context, the mode of modulation is designated A1A.