The United States National Snow and Ice Data Center “Cryosphere Glossary”: http://nsidc.org/cryosphere/glossary

Sea Ice:
Any form of ice found at sea which has originated from the freezing of sea water.

The United States National Snow and Ice Data Center Sea ice FAQ: http://nsidc.org/arcticseaicenews/faq/

Area and extent are different measures and give scientists slightly different information. Some organizations, including Cryosphere Today, report ice area; NSIDC primarily reports ice extent. Extent is always a larger number than area, and there are pros and cons associated with each method.

The United States National Snow and Ice Data Center “State of the Cryosphere” sea ice page: https://nsidc.org/cryosphere/sotc/sea_ice.html

Sea ice is frozen seawater that floats on the ocean surface. Blanketing millions of square kilometers, sea ice forms and melts with the polar seasons, affecting both human activity and biological habitat. In the Arctic, some sea ice persists year after year, whereas almost all Southern Ocean or Antarctic sea ice is “seasonal ice,” meaning it melts away and reforms annually. While both Arctic and Antarctic ice are of vital importance to the marine mammals and birds for which they are habitats, sea ice in the Arctic appears to play a more crucial role in regulating climate.

The United States National Snow and Ice Data Center “All about sea ice” page: https://nsidc.org/cryosphere/seaice/index.html

Because most of us do not live in the polar regions, we may live for several decades and never see sea ice. Although it may not directly affect us, it is a critical component of our planet because it influences climate, wildlife, and people who live in the Arctic.

The NASA Earth Observatory “Sea Ice” page: http://earthobservatory.nasa.gov/Features/SeaIce/

Sea ice is frozen seawater that floats on the ocean surface. It forms in both the Arctic and the Antarctic in each hemisphere’s winter; it retreats in the summer, but does not completely disappear. This floating ice has a profound influence on the polar environment, influencing ocean circulation, weather, and regional climate.