A billboard is a very large outdoor sign designed to catch the attention of people driving or walking by. Most billboards stand along highways or busy streets, displaying advertisements for products, restaurants, movies, or services. They're built to be read quickly, since drivers might pass by in just a few seconds, so they typically use big, bold words and eye-catching images.

Traditional billboards are printed on vinyl or paper and pasted onto wooden or metal frames, though modern digital billboards use giant screens that can display changing images, like enormous television screens outdoors.

Billboard advertisements are expensive because they reach thousands of people every day. Companies pay to rent billboard space for weeks or months at a time, competing for the best locations where the most eyes will see their message. Some billboards have become famous landmarks, like the bright, flashing signs in Times Square in New York City.

You might also hear someone say that an idea or achievement became a “billboard” for something, meaning it publicly represents or promotes that thing to the world.