Also known as: 下敷き
A thin plastic sheet placed under a piece of Paper, either to prevent marking on the sheets below, or to provide a better surface for Writing. Usually referred to as a "pencilboard" in English.
If you rub a shitajiki against someone's head, their hair will be drawn to it due to Static Electricity.
This tag is for shitajiki in the image. For Scans of real shitajiki, which often have pictures of anime characters printed on them, see Shitajiki (Medium).