1. In general, the ratio of the peak variation in the modulation actually used in a transmitter to the maximum variation for which the transmitter was designed.

2. In an amplitude- modulated wave, the ratio (usually expressed in percent) of the peak variation of the envelope from its reference value, to the reference value. Also known as index of modulation.

3. In a frequency-modulated wave, the ratio of the actual frequency swing to the frequency swing required for 100% modulation.