n. a flat (usually rectangular) container for a letter, thin package, etc.
n. any wrapper or covering
n. a curve that is tangent to each of a family of curves
n. a natural covering (as by a fluid)
n. 信封, 封套, 封袋
[医] 膜, 包袋
1. I put the letter in an envelope before mailing it.
我把信放进信封里，然后寄出。
2. Please seal the envelope tightly so it doesn't open.
请把信封密封好，以免打开。
3. She decorated the envelope with colorful stickers.
她用五颜六色的贴纸装饰了信封。