Spectral envelope
A spectral envelope  is the envelope curve of the amplitude spectrum. It describes one point in time (one window, to be precise).

In remote sensing using a spectrometer, the spectral envelope of a feature is the boundary of its spectral properties, as defined by the range of brightness levels in each of the spectral bands of interest.

External links 
 Swartz, Diemo (1998). "Spectral Envelopes". .