envelope
//ˈɛn.və.loʊp//
Noun
envelopeSingular
envelopesPlural
1
A flat paper container with a sealable flap, used to enclose a letter or document.
I put the letter in an envelope and mailed it.
2
A covering or containing structure or layer.
The building's glass envelope helps regulate temperature.
3
The outer casing of a gas balloon or airship.
The envelope of the hot air balloon was made of strong nylon fabric.