{ En"vel*ope (?; 277), En*vel"op (?; 277), } n. [F. enveloppe.] 1. That which envelops, wraps up, encases, or surrounds; a wrapper; an inclosing cover; esp., the cover or wrapper of a document, as of a letter.

2. (Astron.) The nebulous covering of the head or nucleus of a comet; -- called also coma.

3. (Fort.) A work of earth, in the form of a single parapet or of a small rampart. It is sometimes raised in the ditch and sometimes beyond it. Wilhelm.

4. (Geom.) A curve or surface which is tangent to each member of a system of curves or surfaces, the form and position of the members of the system being allowed to vary according to some continuous law. Thus, any curve is the envelope of its tangents.

• A paper or cardboard wrapper used to enclose and small, flat items, especially letters, for mailing.
  
• A bag containing the lifting gas of a balloon or airship.
  
• (geometry) A mathematical curve, surface, or higher-dimensional object that is the tangent to a given family of lines, curves, surfaces, or higher-dimensional objects.
  
• (electronics) A curve that bounds another curve or set of curves, as the modulation envelope of an amplitude-modulated carrier wave in electronics.
  
• (music) The shape of a sound, which may be controlled by a synthesizer or sampler.
  
• (computing) The information used for routing an email that is transmitted with the email but not part of its contents.
  
• (biology) The outer layer of some viruses.
  
• The limits within which an aircraft is permitted to operate.