1.
sealed letter; sealed envelope; letter (in an envelope)
A letter or document enclosed and sealed inside an envelope, as distinguished from an open postcard. Used especially in postal contexts where the format affects postage rates.
A letter or document enclosed in a sealed envelope, as opposed to an open postcard. Composed of 封 (seal) + 書 (document/letter). In postal contexts, 封書 is contrasted with 葉書 (postcard), as they have different postage rates.