Fast ice (also called land-fast ice, landfast ice, and shore-fast ice) is sea ice or lake ice that is "fastened" to the coastline, to the sea floor along shoals, or to grounded icebergs. Fast ice may either grow in place from the sea water or by freezing pieces of drifting ice to the shore or other anchor sites. Unlike drift (or pack) ice, fast ice does not move with currents and winds.
The width (and the presence) of this ice zone is usually seasonal and depends on ice thickness, topography of the sea floor and islands. It ranges from a few meters to several hundred kilometers. Seaward expansion is a function of a number of factors, notably water depth, shoreline protection, time of year and pressure from the pack ice. In Arctic seas the fast ice extends down to depths of 20 m (65.6 ft), while in the Subarctic seas, the zone extends to depths of about 10 m (32.8 ft). In some coastal areas with abrupt shelf and no islands, e.g., in the Sea of Okhotsk off Hokkaidō, tides prevent the formation of any fast ice. Smaller ocean basins may contain only the fast ice zone with no pack ice (e.g. McMurdo Sound in Antarctica).