noun
sealed letter; sealed document
Refers to a letter or document that has been sealed, often in a formal or official context. The gloss 'enclosure' is less common and typically applies when the sealed item is enclosed within something else.
彼は重要な書類を一封にして送った。
He sent the important documents as a sealed letter.
机の上に未開封の一封が置いてあった。
An unopened sealed letter was on the desk.