sea ice can be classified according to whether or not it is attached (or frozen) to the shoreline (or between shoals or to grounded icebergs). if attached, it is called landfast ice, or more often, fast ice (from "fastened"). alternatively, and unlike fast ice, drift ice occurs further offshore in very wide areas, and encompasses ice that is free to move with currents and winds. the physical boundary between fast ice and drift ice is the "fast ice boundary". the drift ice zone may be further divided into a "shear zone", a "marginal ice zone" and a "central pack". drift ice consists of "floes", individual pieces of sea ice or more across. there are names for various floe sizes: "small" – ; "medium" – ; "big" – ; "vast" – ; and "giant" – more than . the term "pack ice" is used either as a synonym to "drift ice", or to designate drift ice zone in which the floes are densely packed. the overall sea ice cover is termed the "ice canopy" from the perspective of submarine navigation.