Other forms: placards; placarded; placarding
A placard is a sign, especially one that advertises something. You might get a summer job standing on the sidewalk, holding a placard that announces a mattress sale — though it may not be your dream job.
A concert poster, a storefront notice, a protest sign carried in a rally — each of these is a type of placard. A placard can be as impermanent as a square of cardboard with a scrawled message, and as official as a permanent plaque marking a historic building. The Old French word plaquier is at the root of placard, and it means "to plaster or to lay flat."