A mass of land ice of continental size (more than 50,000 square kilometres or 19,000 square miles) that is sufficiently thick to cover most of the underlying bed so that its shape is mainly determined by its dynamics (the flow of the ice as it deforms internally and/or slides at its base).
An ice sheet flows outward from a high central ice plateau with a small average surface slope. The margins usually slope more steeply, and most ice is discharged through fast flowing ice streams or outlet glaciers, in some cases into the sea or into ice shelves floating on the sea.
There are only two ice sheets in the modern world, one on Greenland and one on Antarctica. During glacial periods there were others.