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envelope
The noun form usually evokes a sense of privacy and containment, acting as a physical barrier between a secret message and the outside world. It carries a connotation of formality and tradition in an era of digital communication. In technical or aerospace contexts, the term shifts toward a boundary of safety. Here, it describes the absolute threshold of performance beyond which a system fails, turning a physical container into a conceptual limit.
Countable when referring to the paper stationery used for mailing letters. Uncountable when referring to the mathematical or physical boundary of a system's performance limits.
意味
A flat paper container, typically rectangular with a sealable flap, used for enclosing a letter or document.
"She slipped the invitation into the envelope and sealed it."
A wrapping covering; an outer layer that surrounds or contains something.
"The aircraft's flight envelope defines the limits of its speed and altitude."
To wrap up, cover, or surround something completely.
"A thick mist began to envelope the valley as night fell."