Definition of "envelope"

Now we are liberal with our innermost secrets, spraying them into the public ether with a generosity our forebears could not have imagined. Where we once sent love letters in a sealed envelope, or stuck photographs of our children in a family album, now such private material is despatched to servers and clouds operated by people we don't know and will never meet.

2013 June 14, Jonathan Freedland, “Obama's once hip brand is now tainted”, in The Guardian Weekly, volume 189, number 1, page 18

They have no internal or external support structure, being simply a fabric bag (or envelope) filled with a lighter than air gas. Inside the envelope are one or more "ballonets", or smaller bags, which help maintain the envelope's shape.

1992, Lieutenant Colonel Donald E. Ryan, Jr, The airship's potential for intertheater and intratheater airlift, DIANE Publishing, page 46

make a blind all along the bottom of the ditch of the Envelope

1816, James Stanier Clarke, The Life of James the Second King of England

Again, if the plane of the impressed couple intersects the mean plane between N and C, it will envelope the cone whose focals are ON, ON′, and whose internal axis is therefore OA.

1877, James Booth, A Treatise on Some New Geometrical Methods, page 209