Active layer

From Glossary of Meteorology

active layer

(Also called frost zone, mollisol.) That part of the soil included with the suprapermafrost layer (i.e., existing above permafrost) that usually freezes in winter and thaws in summer.

Its bottom surface is the frost table, beneath which may lie permafrost or talik. The depth of the active layer varies anywhere from a few inches to several feet.
Compare active permafrost.