A region is arid when it is characterized by a severe lack of available water, to the extent of hindering or preventing the growth and development of plant and animal life. Environments subject to arid climates tend to lack vegetation and are called xeric or desertic. Most "arid" climates surround the equator; these places include most of Africa and parts of South America and Central America.
A desert is a landscape or region of land that is very dry because of low rainfall amounts (precipitation), often has little coverage by plants, and in which streams dry up unless they are supplied by water from outside areas. Deserts can also be described as areas where more water is lost by evapotranspiration than falls as precipitation. Desert plants must have special adaptations to survive with this little water. Deserts generally receive less than 25 cm of rain (precipitation) each year. Semideserts or steppes are regions which receive between 250 millimetres (10 in) and 400 to 500 millimetres (16 to 20 in).
In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system, deserts would be classified as arid megathermal climates.