Arctic soils are a large group of various soil types formed in extremely cold conditions, with one of their common diagnostic features being the presence of perennially frozen layers known as "permafrost". In addition to permafrost layers, many Arctic soil types also include seasonal freeze-thaw layers that freeze and thaw with the changing seasons. It is possible that certain Arctic soils may not have perennial frozen layers, but still exhibit seasonal freeze-thaw behavior, freezing to a certain depth in winter and thawing in summer, indicating the thickness of the active layer (the layer of soil that thaws seasonally) in present-day soil formation. The process of cryogenesis involves the genesis (formation, development, and evolution) of soils under frozen conditions over many years. The terms "Arctic soils" and "cryogenesis" are often used interchangeably, while the concepts of "cryogenic processes" and "cryogenic phenomena" are widely used to describe long-term interactions of soil and landscapes in cold environments.