A signal which represents the occurance of an event (such as a key being pressed) with a brief pulse, after which the signal returns to its base state. Another way of saying this is that the leading edge of the trigger pulse indicates that an event has occurred, but the trailing edge represents nothing; it is simply necessary to prepare for the next event. Compare with gate.
When an envelope generator has both gate and trigger inputs, the usual behavior is: the leading edge of a pulse at the gate input will initiate the attack phase, and as long as the gate remains high, it will then precede through the decay and whatever other phases exist until it reaches the sustain phase, at which it wil remain as long as the gate remains high. The trailing edge of the gate will initiate the release phase. The leading edge of a pulse at the trigger input will also initiate the attack phase, after which the envelope generator will proceed through all of its phases without interruption, as long as another trigger pulse does not occur. If another trigger does occur, the envelope generator will immediately jump back to the beginning of its attack phase, no matter what phase it is at.