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.
He sent the important documents as a sealed letter.
An unopened sealed letter was on the desk.
monetary gift in an envelope; cash reward
Compound of 一 (one) and 封 (seal), literally 'one seal', referring to a single sealed item.