ADSR is an acronym that stands for Attack, Decay, Sustain, Release and is a means to replicate those respective elements of a sound.It is especially used in sound designing with electronic music instruments.
ADSRsound; often its loudness over time. The envelope may be a discrete circuit or module (in the case of analog devices), or implemented as part of the unit's software (in the case of digital devices).
When a mechanical musical instrument produces sound, the Loudness of the sound produced changes over time in a way that varies from instrument to instrument. When a pipe organ's key is pressed, for example, it plays a note at constant volume; the sound dies quickly when the key is released. By contrast, the sound of a guitar is loudest immediately after a string is plucked, and quickly fades.
A synthesizer's ADSR envelope is a way to tailor the timbre for the synth, sometimes to make it sound more like a mechanical instrument. A quick attack with little decay helps it sound more like an organ; a longer decay and zero sustain makes it sound more like a guitar. While envelopes are most often applied to volume, they are also commonly used to control other sound elements, such as filter frequencies or oscillator pitches.
While ADSR envelopes are a useful first approximation to the volumes of real instruments, they are not a complete substitute. Woodwind and brass instruments give the player the ability to vary the sound arbitrarily throughout a note, for instance. Many synthesizers, therefore, offer more flexible facilities for volume control which can be used if desired.
On older synthesizers, such as the Korg MS-20, a common variation on the ADSR was ADSHR (attack, decay, sustain, hold, release). By adding a "hold" parameter, the system allowed notes to be held at the sustain level for a length of time before decaying. The General Instruments AY-3-8912 sound chip included the hold time only; the sustain level was not programmable. Another common variation in the same vein is the AHDSR (attack, hold, decay, sustain, release) envelope, in which the "hold" parameter controls how long the envelope stays at full volume before entering the decay phase.
The ADSR was first specified by Vladimir Ussachevsky in 1965, then head of the Columbia-Princeton Electronic Music Center, while suggesting improvements for Bob Moog's pioneering work on synthesizers.  An early implementation of ADSR can be found on the Hammond Novachord (which predates Moog synthesizers by several decades).
Inverted ADSR envelope
Some electronic musical instruments allow to invert the ADSR envelope. This results in opposite behaviour compared to the normal ADSR envelope: during the attack phase, the modulated sound parameter fades out from the maximum amplitude to zero, then, during the decay phase, rises up to the value specified by the sustain parameter. After the key has been released, the sound parameter rises from the sustain amplitude back to maximum amplitude.
The contour of the ADSR envelope is specified using four parameters:
How quickly the sound reaches full volume after the sound is activated (the key is pressed). For most mechanical instruments, this period is virtually instantaneous. However, for some popular synthesized voices that don't mimic real instruments, this parameter is slowed down. Slow attack is commonly part of sounds called pads.
How quickly the sound drops to the sustain level after the initial peak.
The "constant" volume that the sound takes after decay until the note is released. Note that this parameter specifies a volume level rather than a time period.
How quickly the sound fades when a note ends (the key is released). Often, this time is very short. An example where the release is longer might be a percussion instrument like a glockenspiel, or a piano with the sustain pedal pressed.