Placard

[Sarah]

An extra plate on the lower part of the breastplate orbackplate. Planché.

[Sarah]

noun: a public [sign], usually paper, or a town notice. also, signs carried or worn by demonstrators or picketers.
verb: to publicize or advertise by usings placards, most commonly used when referring to protestors wearing placards.

[Agnes]

a large piece of card, paper, etc. with a message written or printed on it, often carried in public places by people who are complaining about something:

[Gregory]

A poster or sign for public display, either fixed to a wall or carried during a demonstration.