Fertile tract of land that occurs in a desert wherever a permanent supply of fresh water is available.
Oases vary in size from about 2.5 acres (1 hectare) around small springs to vast areas of naturally watered or irrigated land. Underground water sources account for most oases; their springs and wells are supplied from sandstone aquifers whose intake areas may be more than 500 mi (800 km) away. Two-thirds of the population of the Sahara live in oases, where the date palm is the main source of food; the palm also provides shade for growing citrus fruits, figs, peaches, apricots, vegetables, and cereal grains.