What is a dormant volcano?

There are about 500 active volcanoes around the world, not including those hidden by the sea. A few, such as Stromboli, a volcanic island off the north-eastern coast of Sicily, are active nearly all the time. But most undergo long periods when they are dormant, or sleeping. Some volcanoes stay dormant for so long that they are thought to be extinct and it comes as a terrible shock when they unexpectedly erupt again.