en·ve·lope /ˈɛnvəˌlop, &dɪvɪdɛ;ˈɑn-/
 信封;包裹物,封皮,封套;外殼,殼層;包跡,包線
 en·ve·lope /ˈɛnvəˌlop, ˈɑn-/ 名詞
 外膜(細菌),被膜(病毒),總苞,包絡線,外殼,氣囊
 envelope
 波封; 保護套 EN
 envelope
 包封 包絡
 En·vel·ope En·vel·op n.
 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.
 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.
 push the envelope  to increase the capability of some type of machine or system; -- usually by technological development.
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 envelope
      n 1: a flat rectangular paper container for papers
      2: any wrapper or covering
      3: a curve that is tangent to each of a family of curves
      4: a natural covering (as by a fluid); "the spacecraft detected
         an envelope of gas around the comet"
      5: the maximum operating capability of a system; "test pilots
         try to push the envelope"
      6: the bag containing the gas in a balloon [syn: gasbag]