From Glossary of Meteorology
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  1. (
    Also called perennially frozen ground, pergelisol, permanently frozen ground.) 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.

    Muller, S. W. 1947. Permafrost, or Permanently Frozen Ground, and Related Engineering Problems.

    Hare, F. K. 1951. Compendium of Meteorology. p. 958, and map, p. 956.