"A spectral envelope is a curve in the frequency-amplitude plane, derived from a Fourier magnitude spectrum."[1]
Article: http://recherche.ircam.fr/anasyn/schwarz/da/specenv/3_3Spectral_Envelopes.html
I still don't understand this fully, but it seems to be very closely related to the FFT. I think that you take a FFT of some input signal which yields the spectrum. Then, you generate a curve that fits that spectrum very closely. I assume that at that point you can do some useful math with the generated curve, transforming it or applying it to other signals. Perhaps that is how it fits into the [world-vocoder] which references spectral envelopes heavily.
The [world-vocoder] uses [cheaptrick] to do spectral envelope estimation.
[1] http://recherche.ircam.fr/anasyn/schwarz/da/specenv/3_3Spectral_Envelopes.html