a flat (usually rectangular) container for a letter, thin package, etc.
"She slid the photograph into an envelope before mailing it to her friend."
any wrapper or covering
"The old newspaper was used as an envelope to protect the fragile vase during transport."
a curve that is tangent to each of a family of curves
"The locus of vertices forms an envelope that is tangent to every member of the family of parabolas."
a natural covering (as by a fluid)
"the spacecraft detected an envelope of gas around the comet"
the maximum operating capability of a system (especially an aircraft)
"test pilots try to push the envelope"
A paper or cardboard wrapper used to enclose small, flat items, especially letters, for mailing.
"She carefully placed the letter inside a white envelope before stamping it and dropping it in the mailbox."
In plain English: An envelope is a flat paper bag used to hold and protect letters before you mail them.
"She carefully placed the birthday card into an envelope before mailing it to her friend."
Usage: Use the noun envelope specifically for the outer container that holds mail or documents before they are placed in a mailbox. Do not confuse it with an envelope verb form, which is archaic and rarely used in modern English.
Archaic form of envelop.
"In this archaic text, the author uses the verb envelope to mean that he will completely cover the truth."
The English word comes from the Old French term enveloppe, which originally meant "a covering." While related to an ancient root meaning "to cover," its specific use in engineering for a flight's performance limits was developed much later, and the common verb form is simply derived from this noun.