envelope
nounUK /ˈenvələʊp/
US /ˈenvəloʊp/
A paper cover that holds a letter or card.
She put the birthday card in an envelope and mailed it.
OTHER MEANINGS
2. The outer covering of a machine or object.
The envelope of the balloon protected the gas inside.
3. The limits of what a system or vehicle can safely do.
The pilot pushed the plane to the edge of its flight envelope.
4. A curve that touches all sides of a shape.
In math class, we drew the envelope around the triangle.