means A large printed sheet meant to be displayed on a wall, used to advertise, inform, decorate, or rally support around a cause, event, or person.

from From "post," the upright stake or pillar where public notices were once nailed up for all to read — a word that traces back through Middle English to the Latin "postis," a doorpost or upright beam. A poster, then, was literally something put up on a post; the meaning drifted from the wooden support to the paper it carried, and finally to any bold sheet pasted in public view.