Traditionally, the active layer has been defined as the surficial layer above permafrost which thaws during summer. Under this definition, the active layer and permafrost are distinct. The definition has been used almost universally since the term was introduced into the English language by S. W. Muller over half a century ago. The definition of the active layer presented recently in the Canadian Glossary of Permafrost and Related Ground-Ice Terms is based exclusively on the freezing of water in soil. The definition is constructed such that active-layer thickness cannot be determined consistently in space and time, and is, therefore, impossible to use effectively in the field.

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Permafrost and Periglacial Processes
Department of Geography and Environmental Studies

Burn, C. (1998). The active layer: Two contrasting definitions. Permafrost and Periglacial Processes, 9(4), 411–416. doi:10.1002/(SICI)1099-1530(199810/12)9:4<411::AID-PPP292>3.0.CO;2-6