Every volcano has its own eruptive history, and most can be classified into three main types that are based on the volcano's general form and eruptive pattern. Categorizing a volcano as active, dormant or extinct is one way of acknowledging its eruptive history, though these categories are necessarily inexact and subjective.
A volcano is considered active if it has erupted within known history, which, according to the Global Volcanism Program, is usually defined as within the past 10,000 years or since the Ice Age. Volcanoes that are currently erupting or have shown signs of unrest, such as gas discharges or triggering earthquakes, are also classified as active volcanoes.
A dormant volcano is one that has not erupted for the past 10,000 years but could erupt again. Once a dormant volcano erupts, it then falls into the category of active volcano. The Yellowstone Caldera in Yellowstone National Park, for example, has not erupted for 600,000 years as of 2014, but scientists believe it is a sleeping or dormant volcano and could still erupt in the future.
An extinct volcano, on the other hand, is one that experts don't expect to erupt ever again because it has been cut off from its supply of magma. Classic examples of extinct volcanoes include the chain that created the Hawaiian Islands.