According to Volcano World, a website sponsored by the Department of Geosciences at Oregon State University, a dormant volcano is a volcano that has not erupted in 10,000 years but is expected to eventually erupt again. A volcano is classified as dormant, active or extinct.
Active volcanoes have erupted in the past 10,000 years and may even be erupting in present day. Extinct volcanoes are not expected to ever erupt again. According to Universe Today, a volcano becomes extinct when the tectonic plate it sits upon shifts away from a volcanic hotspot beneath the Earth's crust and no longer has access to a magma chamber stored below.