noun
sticky note; tag; label; Post-it note
Refers to small adhesive notes used for reminders, labels, or bookmarks. 付箋 is the general term; ポストイット is the trademarked brand name.
大事なページに付箋を貼った。
I put a sticky note on the important page.
冷蔵庫に買い物リストの付箋を貼っておく。
I stick a shopping list sticky note on the fridge.
付 (attach) + 箋 (slip of paper). The spelling 附箋 uses the older variant 附 for 付.