Envelope is a time-based control that traditionally controls the amplitude of a synthesiser oscillator over time. Most envelopes are divided into four controls, often abbreviated as ADSR: Attack, Decay, Sustain, Release. Attack is to determine how long it takes for the loudness to reach its maximum value and Decay is to determine the curve of how the loudness decreases after reaching its maximum value. Sustain starts after the end of decay and defines how long the amplitude of the end of decay lasts. Release is another decay curve that is only triggered after a key has been released on the keyboard. In the alchemy synth in logic, there is also an H for Hold time, which allows you to adjust the amount of time the peak amplitude level is held before the envelope decay phase begins.

Multi-stage envelopes can be found for complex modulation possibilities compared to simple two- or four-stage envelopes.

The typical use of an envelope is to control or modulate the amplitude. But the difference between using an LFO and an envelope modulation parameter is that an LFO is intended to be repeated continuously at a specific rate or frequency, whereas an envelope is usually based on an event triggered by some user input (keystroke). The envelope generates data based on the duration of the event.