envelope

envelope
/ˈenvələup, alsoˈɔn-; `ɛnvəˌlop, `ɑnvəˌlop/ n
wrapper or covering, esp one made of paper for a letter 封套; 封皮; （尤指）信封
    writing paper and envelopes 信纸和信封
    an airmail envelope 航空信封.

牛津英汉双解词典