Meaning of ENVELOPE in English
noun Date: circa 1714 a flat usually paper container (as for a letter), something that envelops ; wrapper , 3. the outer covering of an aerostat, the bag containing the gas in a balloon or airship, 4. a natural enclosing covering (as a membrane, shell, or integument), a lipoprotein unit membrane that forms the outer layer of some virions, 5. a curve tangent to each of a family of curves, a surface tangent to each of a family of surfaces, a set of performance limits (as of an aircraft) that may not be safely exceeded, a conventionally accepted limit , Usage: The \\ˈen-\\ and \\ˈän-\\ pronunciations are used with about equal frequency, and both are fully acceptable, though the \\ˈän-\\ version is sometimes decried as “pseudo-French.” Actually \\ˈän-\\ is exactly what one would expect to hear when a French word like entrepreneur is becoming anglicized. Envelope, however, has been in English for nearly 300 years, plenty of time for it to become completely anglicized and for both of its pronunciations to win respectability.
Merriam Webster. Explanatory English dictionary Merriam Webster. Толковый словарь английского языка Мерриам-Уэбстер. 2012