From Envirowiki

A desert is a ecosystem with very little water. Often, deserts have very little in the way of ground cover, with bare earth, or sand. They are harsh ecosystems, but this does not mean they have no biodiversity. They often have a small diversity of species, with large genetic differences between them, and with a high low resilience to ecosystem shocks, due to short and uncomplex food chains.

The polar regions of earth are both deserts, not because there is no water, but because the water is nearly all frozen (no liquid water). Antarctica is the world's largest desert. The Sahara is the world's largest hot desert.