The envelope of an e-mail, which contains the information used for routing

Two envelope problem, a paradox

Envelope, the paper container used to hold a letter being sent by post

envelope detector, in electronics, an envelope is a curve that bounds another curve or set of curves, as the modulation envelope of a carrier wave

Envelope (band) -- a pop/rock band from Denmark.

ADSR envelope, the variation of a sound over time, as is used in sound synthesis

Double envelopment (Pincer movement).

Envelopes (band) -- an indie/pop band from Sweden and France, based in the UK.

envelope (mathematics) -- a curve, surface, or higher dimensional object defined as being tangent to a given family of lines or curves (or surfaces, or higher dimensional objects, respectively).

envelope (mechanical engineering) -- a solid representing all positions that an object may occupy during its normal range of motion.

building envelope -- the exterior layer of a building that protect it from the elements.

envelope (aerospace) -- the limits within which an aircraft can operate.

Cyclopentane, a chemistry term meaning the three-dimensional conformation of is commonly referred to as the "envelope" structure.

Viral envelope is biological term meaning a membranal covering surrounding the capsid