Refers to a letter that is sealed in an envelope, as opposed to a postcard or open correspondence. Often used in official or formal contexts.
Please send important documents by sealed letter.
A sealed letter arrived, so I opened it immediately.
I sent the important documents as a sealed letter.
I decided to send it as a sealed letter instead of a postcard.
はがき is an open postcard with no envelope, whereas 封書 is a letter sealed inside an envelope for privacy.
封筒 is the physical envelope itself, while 封書 refers to the whole sealed letter (envelope plus contents) sent through the mail.
expert; connoisseur; person in the know · counter for messages and documents · worldly understanding; tact; insight · supernatural powers; magical powers
Compound of 封 (seal) and 書 (writing/document). The combination straightforwardly means a sealed document or letter.