envelope

/en"veuh lohp', ahn"-/, n.

1. a flat paper container, as for a letter or thin package, usually having a gummed flap or other means of closure.

2. something that envelops; a wrapper, integument, or surrounding cover.

3. Biol. a surrounding or enclosing structure, as a corolla or an outer membrane.

4. Geom. a curve or surface tangent to each member of a set of curves or surfaces.

5. Radio. (of a modulated carrier wave) a curve connecting the peaks of a graph of the instantaneous value of the electric or magnetic component of the carrier wave as a function of time.

6. the fabric structure enclosing the gasbag of an aerostat.

7. the gasbag itself.

8. Electronics. the airtight glass or metal housing of a vacuum tube.

9. the technical limits within which an aircraft or electronic system may be safely operated.

10. push the envelope, to stretch the limits, as of technological advance, social innovation, etc.

Also, envelop.

[1700-10; < F enveloppe, deriv. of envelopper to ENVELOP]

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▪ mathematics

      in mathematics, a curve that is tangential to each one of a family of curves in a plane or, in three dimensions, a surface that is tangent to each one of a family of surfaces. For example, two parallel lines are the envelope of the family of circles of the same radius having centres on a straight line. An example of the envelope of a family of surfaces in space is the circular cone x² − y² = z² as the envelope of the family of paraboloids (paraboloid) x² + y² = 4a(z −  a).

▪ poetry

      in poetry, a device in which a line or a stanza is repeated so as to enclose a section of verse, as in Sir Thomas Wyatt (Wyatt, Sir Thomas)'s “Is it Possible?”:

Is it possible

That so high debate,

So sharp, so sore, and of such rate,

Should end so soon and was begun so late?

Is it possible?

▪ sound

      in musical sound (music), the attack, sustain, and decay of a sound. Attack transients consist of changes occurring before the sound reaches its steady-state intensity. Sustain refers to the steady state of a sound at its maximum intensity, and decay is the rate at which it fades to silence. Envelope, the combination of the three components of a dynamic musical tone, is an important element of timbre, the distinctive quality, or tone colour, of a sound. Every musical instrument has its characteristic attack, sustain, and decay pattern. Attack transients are very complex and difficult to characterize because of the speed with which the character of the sound changes in its first few milliseconds, and they are the subject of current research into exactly how they affect the tone quality of musical instruments.

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