envelope
noun /ˈenvələʊp/, /ˈɒnvələʊp/
/ˈenvələʊp/, /ˈɑːnvələʊp/
Idioms 
enlarge imagea flat paper container used for sending letters in, with a part that you stick down to close it- Writing paper and envelopes are provided in your room.
- a padded/prepaid envelope
Extra Examples- He gave her the envelope of certificates.
- He quickly stuffed the money back into the envelope.
- I couldn't read the address on the envelope.
- I had put the letter in the wrong envelope.
- I scribbled his phone number on the back of an envelope.
- Please enclose a self-addressed envelope if you would like a reply.
- a self-addressed stamped envelope
- The envelope was marked ‘Personal’.
- The letter was suspicious, and I considered steaming open the envelope.
- To apply, use the enclosed reply-paid envelope (no stamp needed).
- You don't earn much stuffing envelopes.
- an envelope addressed in my mother's round handwriting
- an official-looking manila envelope
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
- Each item was tagged and sealed in a plastic envelope.
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
- We added up our savings on the back of an envelope.
- back-of-the-envelope calculations
push the envelope
- (informal) to go beyond the limits of what is allowed or thought to be possible
- He is a performer who consistently pushes the envelope of TV comedy.