1A paper envelope used to hold letters or documents; also used for mailing, with an address written on it and a stamp attached.
コンビニで封筒とはがきを買った。
I bought envelopes and postcards at the convenience store.
封筒に宛名を書いて、切手を貼った。
I wrote the recipient's name on the envelope and put a stamp on it.
大事な書類なので、厚めの封筒に入れて送った。
Since it was an important document, I sent it in a thicker envelope.