Blu-tack is a soft, sticky lump — a bit like very firm chewing gum — that you press onto the back of a poster or a card to hold it on a wall. You can pull it off later and use it again, and it should not take the paint or wallpaper with it. It began as a brand name; the original stuff is blue, and tack here is the sticky hold. In Britain, Australia, and New Zealand people use Blu-tack for the stuff in general, even other brands. You can also blu-tack something to a wall, as a verb. Though actually, American English more often says poster putty or sticky tack. In a Taiwan classroom this is simply the English for 藍丁膠.