Similar term(s): pergelisol.
A layer of soil or bedrock at a variable depth beneath the surface of the
earth in which the temperature has been below freezing continuously from a few
to several thousands of years.
While the top layer of soil heats up in the summer, permafrost exists in
deeper layers where the summer heating fails to melt the frozen ground. A
continuous layer of permafrost is found in areas where the annual mean
temperature is below about −5°C (23°F).
Source: based on AMS Glossary of Meteorology
To read about this term in context:
GreenFacts Summary on Climate Change 2007 Update: