From AMS Glossary
Soil within which the moisture has predominantly changed to ice, the unfrozen portion being in vapor phase.
Ice within the soil bonds (adfreezes) adjacent soil particles and renders frozen ground very hard. "Permanently" frozen ground is called permafrost. "Dry" frozen ground is relatively loose and crumbly because of the lack of bonding ice. Frozen ground is sometimes inadvisedly called frost or ground frost.