The Jolly Roger is the flag pirates used to identify themselves. The iconic, traditional Jolly Roger is a black flag with a white skull and crossbones.
In the 18th century, most pirates identified themselves at sea by flying the Jolly Roger, especially when they were poised to attack another ship. The name first appeared in print in a popular 1724 British book about pirates. While many of these oceangoing outlaws apparently used the term for their intimidating-looking flags, its origin is uncertain — though at the time jolly had the alternative meaning "gallant or bold."