# envelope

/ˈɛnvəˌɫoʊp/ (US)

Source: https://englishreference.com/word/envelope
Updated: 2026-06-20

## Senses

### 1. for letters

noun · CEFR A2

a flat paper container used for mailing letters or cards.

For teachers: a flat, usually rectangular paper container with a sealable flap, used for enclosing letters or documents for post.

- She licked the back of the envelope and pressed it shut.
- The solicitor handed over a thick manila envelope containing the original deeds to the property.

### 2. technical limits

noun · CEFR C1 · Technical

the limits of what a machine or system can safely do.

For teachers: the set of performance limits within which a technological system, especially an aircraft, can operate safely.

- The test pilot was careful not to fly outside the safety envelope.
- By incorporating carbon-fibre components, the design team managed to push the performance envelope of the prototype significantly.

Usage: Commonly used in the idiom 'push the envelope' to mean testing the limits of what is possible.

### 3. outer covering

noun · CEFR B2 · Academic

a layer that wraps around or covers something else.

For teachers: a natural or artificial covering that encloses an object or structure.

- The virus is protected by an outer envelope made of proteins.
- The building's thermal envelope was upgraded with high-performance insulation to reduce energy loss during the winter months.

### 4. sound shape

noun · CEFR C2 · Technical

the way a sound changes in volume over time.

For teachers: the variation in the amplitude of a sound signal over time, typically defined by attack, decay, sustain, and release.

- He adjusted the envelope on the synthesizer to make the notes sound softer.
- The sound engineer manipulated the ADSR envelope to give the lead synth a more percussive, aggressive character.

### 5. balloon bag

noun · CEFR C1 · Technical

the large fabric bag that holds the gas in a hot-air balloon.

For teachers: the fabric structure of an aerostat that contains the lifting gas.

- The ground crew checked the envelope for small holes before the flight.
- The hot-air balloon's nylon envelope expanded rapidly as the burners blasted heat into the opening.

## Examples

- She licked the envelope and stuck a stamp on it.
- The lawyer handed over a thick white envelope containing the signed contracts and the keys to the property.
- While the primary sense refers to postal stationery, engineers often speak of the flight envelope to describe the maximum speed and altitude limits within which an aircraft can safely operate.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.