An "envelope" is a flat, usually rectangular paper cover that is used to enclose and protect letters or documents before they are sent through the mail. Full definition
Earlier this year, I heard a great expression on the radio: Your air conditioner is the faucet; the building envelope of your home is the cup. (energyvanguard.com)
Policy documents are not meant to be kept in the sealed envelope in your closet. (coverfox.com)
Please include a self - addressed, stamped envelope with your request. (animalleague.org)