Dormant volcano

Dormant volcanos are the volcanos that are quiet but might erupt again in the future. If a volcano that has been silent for years erupts again or shows activity, it is called a "dormant volcano". Vesuvius of Italy is a prime example. After many years of dormancy, it re-emerged in 1931. Then it is dormant again. For example, Krakatoa in Indonesia, Aconcagua in Chile, and Mount Fuji in Japan are dormant volcanoes.
Difficult to classify.
It is difficult to distinguish active and inactive volcanos.
For example, some scientists have described Mount Fuji as dormant or inactive. That volcano has been dormant since its last eruption in 1707, but is also classified as active. The Geological Survey of Japan lists Mount Fuji as among the active volcanoes on the island of Honshu. 
Mis-labelled as dormant or extinct.
Some volcanoes may remain dormant for a long time; and an eruption is a surprise. 
Before 79 AD, Vesuvius was considered dormant.
Before 1991, the Pinatubo volcano in the Philippines was labelled as dormant.
Before 1995, the Soufriere Hills volcano on the island of Montserrat was dormant.
Before 1980, Mt. St. Helens in the United States was dormant.
Select list.
There are many examples of volcanoes which are considered dormant.