envelope

A flat paper cover that holds a letter or card, especially when you send it by post.

• Put the letter in an envelope.
• She wrote my address on the envelope.
• Who sent this empty envelope?

Common phrases: sealed envelope, open an envelope, envelope address, paper envelope

A layer or outer cover that surrounds something completely.

• A protective envelope surrounds the cell.
• The balloon’s envelope was filled with hot air.
• The planet has a thin envelope of gas.

How to use it: an envelope of something (An envelope of hot gas surrounds the star.)

Similar words: covering, outer layer

Common phrases: outer envelope, protective envelope, envelope of gas

The limits within which a machine or system can work safely and properly.

• The aircraft stayed within its flight envelope.
• Engineers tested the motor’s operating envelope.
• Flying outside the envelope can damage the aircraft.

How to use it: within/outside the envelope (The pilot kept the aircraft within its flight envelope.)

Similar words: operating limits

Common phrases: flight envelope, operating envelope, performance envelope