envelope

 
Pronunciation: /ˈɛnvələʊp, ˈɒn-/

noun

  • 1a flat paper container with a sealable flap, used to enclose a letter or document.
  • 2a covering or containing structure or layer: the external envelope of the swimming pool
  • the outer metal or glass housing of a vacuum tube, electric light, etc.: the switch itself is a glass envelope filled with rare gases and containing two contacts
  • the structure within a balloon or non-rigid airship containing the gas.
  • Microbiology a membrane which forms the outer layer of certain viruses.
  • Electronics a curve joining the successive peaks of a modulated wave.
  • Mathematics a curve or surface tangent to each of a family of curves or surfaces.

Phrases

the back of an envelope

used in reference to calculations or plans of the most sketchy kind: a proposal drawn up on the back of an envelope

push the envelope (or the edge of the envelope)

approach or extend the limits of what is possible: these are extremely witty and clever stories that consistently push the envelope of TV comedy
[originally aviation slang, relating to graphs of aerodynamic performance]

Origin:

mid 16th century (in the sense 'wrapper, enveloping layer'; originally as envelope): from French enveloppe, from envelopper 'envelop'. The sense 'covering of a letter' dates from the early 18th century