The ADSR Envelope is component of syntheizers used to various parameters including volume, filter cutoff etc. The letters in 'ADSR' stand for Attack, Decay, Sustain, and Release respectively.
Attack: Attack is the time the parameter takes to reach its peak after being triggered.
Decay: Decay is the time the parameter takes to reach its sustain level.
Sustain: Sustain is the time for which the parameter stays constant after decaying but before the release.
Release: Release is the time the parameter takes to drop off to nil again.
There are two types of slopes that the attack, decay and release times will observe; linear and exponential. These different slopes can be attributed to hardware envelope generators and their capacitor charging methods. A voltage source charging the capacitors of a hardware envelope generator will result in an exponential slope, while a constant current source will result in a linear slope. Both methods have their merit, and software envelopes will observe either depending on the creators intent.