Sea ice is any kind of ice found at sea which has originated from the freezing of sea water.

Sea ice comes in a vast array of forms, and can be broadly described as new ice, young ice, and old ice. These categories reflect the age of the ice and include different forms and thicknesses of ice at various stages of development.

The primary forms of sea ice in Antarctica are drift or pack ice, and fast ice. Pack ice is highly mobile and dynamic, and moves with the wind and sea currents, forming the majority of the ice that occurs in a wide band around the continent. Pack ice can develop into floes, icebergs and other mobile forms, with a range of sizes, ages and thicknesses, present in varying concentrations, due to the highly variable ice conditions of the region.

Fast ice, which is ice thicker than 2 m, forms ‘land fast’ phenomena such as ice shelves and grounded ice. Even in winter only a small fraction of sea ice close to the Antarctic continent occurs as a continuous and uniform sheet. This ‘land fast’ ice is pinned to the coast and does not move.