The phrase "outer envelope" refers to the outermost part of a package, letter, or document. It is the layer that protects and holds the contents inside. Full definition
It failed to pin down the day length due to lingering difficulties in measuring the rotation rate of the planet's innards beneath the thick outer envelope of gas. (scientificamerican.com)
The first immunogens are those similar to the trimer outer Envelope protein of native HIV. (iavireport.org)
Large ripples, loops and arcs embedded in the starry outer envelope were first observed in the 1970s, and they remain an active field of study for contemporary astronomers, who use the latest telescope technology to observe the finer details of NGC 1316's unusual structure through a combination of imaging and modelling. (sciencedaily.com)