An excellent, comprehensive glossary of terms related to the cryosphere can be found at the National Snow and Ice Data Center website.
Firn: rounded, well-bonded snow that is older than one year; firn has a density greater than 550 kilograms per cubic-meter (35 pounds per cubic-foot); called névé during the first year.
Glacier: a mass of ice that originates on land, usually having an area larger than one tenth of a square kilometer; many believe that a glacier must show some type of movement; others believe that a glacier can show evidence of past or present movement.
Ice Cap: a dome-shaped mass of glacier ice that spreads out in all directions; an ice cap is larger than an icefield but less than 50,000 square kilometers (12 million acres)
Ice Sheet: a dome-shaped mass of glacier ice that covers surrounding terrain and is greater than 50,000 square kilometers (12 million acres), i.e., the Greenland and Antarctic ice sheets
Ice Shelf: portion of an ice sheet that spreads out over water, but is still attached to ice sheet, i.e. an ice shelf contains ice that originated in the ice sheet; it will also have new snow accumulation on top and possible marine ice accumulated below. Ice shelves usually occupy coastal embayments, may extend hundreds of km from land and reach thicknesses of 2500 m.
Iceberg: piece of ice that has detached from the terminus of a glacier, ice cap, or ice shelf and floats in the water.
Ice Stream: a current of ice in an ice sheet or ice cap that flows faster than the surrounding ice.
Permafrost: layer of soil or rock, at some depth beneath the surface, in which the temperature has been continuously below 0°C for at least several years; it exists where summer heating fails to reach the base of the layer of frozen ground.
Sea Ice: any form of ice found at sea which has originated from the freezing of sea water; sea-ice floats on an ocean.
Snow (1) an ice particle formed by sublimation of vapor in the atmosphere (2) a collection of loosely bonded ice crystals deposited from the atmosphere; high density snow (greater than 550 kilograms per cubic meter; 34 pounds per cubic foot) is called firn if it is older than one year.