envelope
n.
An enclosure for a flat or folded document, especially one used for sending mail.
A digital entity used for sending electronic mail or messages, consisting of a header for addressing and defining the contents of a message and a body for containing a message.
Notes
An envelope1 is typically made of paper, folded and glued so that one side remains open to allow a flat document to be inserted, possibly after being folded, and with a flap that allows the envelope to be sealed. Standard size envelopes include letter (approximately 4 × 6 inches, 10 × 15 cm) and legal or C10 (approximately 4 × 9 inches, 10 × 23 cm).
