For a pulse wave, the percentage of the cycle in which the wave is in the "high" or "on" state. A square wave is a pulse wave with 50% duty cycle. When a pulse wave is used as an audio waveform, moving the duty cycle away from 50% (in either direction) increases the prominence of the higher harmonics, and reduces the intensity of the fundamental, resulting in a "thinner" sound. (And, if the waveform is being used at bass frequencies, a noticeable repeating "tick" or stuttering in the sound.) If the pulse wave is used as a repeating gate, the duty cycle determines the "gate on" time, between the attack and the release. If the duty cycle is near 100%, the gate will be "on" for most of the cycle; if the duty cycle is near 0%, the gate will be "off" for most of the cycle.