n.[C]

1. 信封；封皮；封套
   Mother addressed the envelope.

   母親在信封上寫姓名地址。

2. 外殼，殼層
3. （氣艇，氣球等的）氣囊

n.

1. a flat paper container with a sealable flap, used to enclose a letter or document.
2. a covering or containing structure or layer.
3. Electronics  a curve joining the successive peaks of a modulated wave.
4. Mathematics  a curve or surface tangent to each of a family of curves or surfaces.