ADSR (attack decay sustain release)  

attack decay (initial) sustain & release (final decay)
ADSR is a simple reduction of what is generally a more complex dynamic envelope.  In synthesizers or samplers it may be used to control amplitude, filter cut-off, or tuning (to name a few likely parameters).  In older analogue synthesizers, the ADSR created a transient control voltage which was used in conjunction with a voltage controlled amplifier, filter or oscillator. It required a trigger to turn it on.  In more contemporary models it often refers to both the envelope and the process.    In most applications the attack, decay and release segments have preset durations while the sustain segment gets its duration from the trigger.  This is based on the model of an organ which will sustain as long as the key is depressed.
    (1) short attack, short decay (organ-like)
    (2) longer attack, short decay
    (3) longer attack, long decay