a flat paper container used for sending letters in, with a part that you stick down to close it 信封 
Writing paper and envelopes are provided in your room. 你的房间里有信纸和信封。 a padded/prepaid envelope 填充/预付信封
Extra ExamplesHe gave her the envelope of certificates. 他给她证书的封套。 He quickly stuffed the money back into the envelope. 他迅速把钱塞回信封里。 I couldn't read the address on the envelope. 我看不清楚信封上的地址。 I had put the letter in the wrong envelope. 我把信放错了信封。 I scribbled his phone number on the back of an envelope. 我在一个信封的背面草草记下了他的电话号码。 Please enclose a self-addressed envelope if you would like a reply. 想要收到回复者，请附寄一个带回信地址的信封。 a self-addressed stamped envelope 一个贴足邮票并写明回邮地址的信封 The envelope was marked ‘Personal’. 信封上标有“私人信件”字样。 The letter was suspicious, and I considered steaming open the envelope. 这封信有点儿可疑，我认为应该用蒸汽拆封。 To apply, use the enclosed reply-paid envelope (no stamp needed). 申请可用随附的已付费信封。 You don't earn much stuffing envelopes. 将信封里塞得满满的不会给你带来多大的好处。 an envelope addressed in my mother's round handwriting 用我母亲的圆体字迹书写地址的信封 an official-looking manila envelope 一个看起来很正式的马尼拉纸信封
Oxford Collocations Dictionaryadjective- bulky
- fat
- thick
- …
- open
- rip open
- slit open
- …
- contain something
- enclose something
- in an/the envelope
- into an/the envelope
- on an/the envelope
- …
- the back of an envelope
a flat container made of plastic for keeping papers in 塑料封套；塑料封皮 Each item was tagged and sealed in a plastic envelope. 每件物品都贴了标签，封进了塑料封套。
Word Originmid 16th cent. (in the sense ‘wrapper, enveloping layer’; originally as envelope): from French enveloppe, from envelopper ‘envelop’. The sense ‘covering of a letter’ dates from the early 18th cent.
Idioms
(on the) back of an envelope
used to talk about planning or calculating something in a way that is not detailed, exact or complete 信封： 用于谈论以不详尽，不准确或不完整的方式规划或计算事物We added up our savings on the back of an envelope. 我们在信封背面写上了我们的存款。 back-of-the-envelope calculations 粗略计算
push the envelope