A thumbtack is that little pin with a wide flat head that you push into a board or a wall with your thumb, to hold up a notice, a photo, or a map. The flat top is there so your thumb has something to push against. This is the American word. In Britain the same object is called a drawing pin, and a British speaker may not know 'thumbtack' at all. Americans also say 'push pin', though a push pin usually has a taller plastic head you can grip, the kind you see stuck in maps in films. It is a countable noun: one thumbtack, three thumbtacks. It works as a verb too — 'she thumbtacked the poster to the wall' — though most people would just say 'pinned it up'. If you want the picture that sticks: it is named for the finger that does the work.

圖釘（美式說法）。英式英文說 drawing pin。台灣教室牆上、佈告欄上用的那種。