1. 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. Permafrost exists where the summer heating fails to descend to the base of the layer of frozen ground. A continuous stratum of permafrost is found where the annual mean temperature is below about −5°C (23°F). Compare pereletok; See active layer. 2. As limited in application by P. F. Svetsov, soil that is known to have been frozen for at least a century.