placard
Placard is a small and usually impermanent notice, or sign. A placard is usually flat, small enough to be carried around, and commonly used in public spaces as a form of advertising. Most placards do not have a signpost, or a method of self-support. The root of the term comes from the old French word plaquier, which means to plaster, or to lay flat.
The term can refer to any paperboard, or similar material sign, yard sale / garage sale / for sale signs, place card / table tent cards, most kinds of hanging signs, and hazard identification placards.