The term supervolcano has no specifically defined scientific meaning. It was coined by the producers of a BBC Popular Science programme in 2000 to refer to volcanoes that have generated Earth's largest volcanic eruptions. As such, a supervolcano would be one that has produced an exceedingly large, catastrophic explosive eruption and a giant caldera.
Because there is no well-defined minimum size for a "supervolcano," there is no exact number of such volcanoes. Examples of volcanoes that produced exceedingly voluminous pyroclastic eruptions and formed large calderas in the past 2 million years would include Yellowstone, Long Valley in eastern California, Lake Toba in Indonesia, and Lake Taupo in New Zealand. Other "supervolcanoes" would likely include the large caldera volcanoes of Japan, Indonesia, Alaska (e.g. Aniakchak, Emmons, Fisher), and other areas. It is believed that such eruptions would fill the earth's atmosphere with ash, blocking out sunlight and causing famine.
Another type of eruption that can be classified as a "supervolcano" is the basaltic flood variety that formed the Deccan Traps in India and the Columbia River basalts of the northwestern United States. These volcanoes probably did not erupt as explosively as the caldera-forming ones, but instead flooded vast regions with thick layers of lava in a short period of time.