\En"vel*ope\ (?; 277), Envelop \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.