envelope

/ˈɛnvəˌloʊp/

a flat (usually rectangular) container for a letter, thin package, etc.

中文释义：信封；封套

She sealed the envelope and put it in the mailbox.

她封好信封，把它投进了邮箱。

The spacecraft's heat shield protects it from the envelope of hot gases.

航天器的隔热罩保护它免受热气层的伤害。