Fetching details from the dictionary...

writing paper; stationery; notepaper
花柄の便せんに手紙を書いて送った。
I wrote a letter on floral stationery and sent it.
白い便せんにペンで丁寧に文章を綴った。
I carefully wrote sentences in pen on white writing paper.
便せんが切れたので文房具店で買ってきた。
I ran out of writing paper, so I bought some at the stationery store.