The following is an excerpt from Wikipedia :
A dormant volcano is a volcano which is not currently active (that is, not erupting nor showing signs of unrest), but is believed to be still capable of erupting. This contrasts with an extinct volcano, where it is believed that no eruptions will occur for the foreseeable future. The term is also used to refer to dormant volcanic fields such as the Auckland volcanic field.
In practice, it is often impossible to distinguish between a dormant and an extinct volcano and several volcanoes thought to be extinct have subsequently re-erupted. Volcanoes can be dormant for hundreds or thousands of years and, due to the lack of eruptions, are usually eroded and worn down. The eruptions from a dormant volcano are usually very violent because the plug inside the volcano stops the lava from coming out of the vent for a very long time, thus building pressure. The volcano will then erupt again and be classified as an active volcano.
The volcano can be referred to as "sleeping," since it is not expected to erupt anytime in the near future, but it is said to erupt soon, within the next 100 years. It is quite possible, however, that the volcano will eventually be active again. Dormant volcanoes sometimes have hot springs and fumarolic activity, when steam and hot volcanic gases, such as sulfur dioxide, are emitted.This may account for many of the sulfurous smells being emitted from the New Scottish Parliament Building (shown above). It has been built in Holyrood Park at the base of Arthur's Seat. Until now the smell of rotten eggs had been unjustly attributed to certain flatulent members of the Scottish Parliament.
For further reading, please follow this link: Housing Prices Lowered by Evidence of Live Volcano