The envelope of a pattern includes any cell touched during the evolution of that pattern. For an oscillator, it is equivalent to the totality of cells making up that oscillator, i.e. the union of its rotor and stator.
The envelope corresponds to the state-2 blue cells in LifeHistory.
The reaction envelope is the collection of cells that are alive during some part of a given active reaction. This term is used for Herschel circuits and other stable circuitry, whereas construction envelope is specific to recipes in self-constructing circuitry.
There are some subtleties at the edges of the envelope. Specifically, two reactions that have the exact same set of cells defining their envelopes may have different behavior when placed next to a single-cell protrusion like the tail of an eater 1, or one side of a tub. The difference depends on whether two orthogonally adjacent cells at the edge of the envelope are ever simultaneously alive, within the protruding cell's zone of influence.