Pack ice is high-concentration, consolidated sea ice that drifts on top of the ocean under the influence of winds and currents, independent of any landmass. Pack ice differs from sea ice in the Marginal Ice Zone, which has low-concentration sea ice and ice floes. Pack ice also differs from land-fast ice connected to a coastline and therefore unable to move with winds or currents. 

Areas of pack ice grow in the winter and shrink in the summer. While the warming of the ocean and atmosphere is melting much of Earth's ice, pack ice is increasing in some areas because warmer waters are detaching sheets of ice from land.

Pack ice is a crucial part of polar ecosystems. For instance, it provides protection for krill to breed without the danger of predators. On the other hand, large, thick masses of pack ice that persist over long periods prevent some animals from accessing the krill, which can lead to a decline in the populations of birds and other species that need krill for food.

Arctic shipping routes are also affected by pack ice, with thick ice necessitating the use of icebreaking ships to clear the way. Because pack ice drifts around independent of land, it can cause unexpected hazards for ships navigating this area of the ocean.

NASA collects data to track the formation and growth of pack ice, providing a useful resource for studies of polar ecosystems, shipping transport, and global warming.