Short definition of “oasis”, written in 1998 for an encyclopedia (at the request of Pr. Raymond Pujol).
Oasis: an oasis is the combination of human settlement and a cultivated area (often a palm grove) in desert or semi-desert environment. An oasian palm grove is a highly anthropic (so artificial) and irrigated space with an agriculture that is classically intensive polyculture or mix-crop farming. The oasis is integrated to its desert environment (combination with nomadic transhumance), but frees itself also thanks to a particular ecosystem structure. This integrated agriculture is organized to meet the environmental constraints thanks to the superposition (in its typical form) of three strata: the tallest of the palm trees, the intermediate of the fruit trees (orange, banana, pomegranate, apple, etc..) and under their shadow the lowest strata of the herbaceous plants (vegetables, fodder, cereals). Another constant of the oasis structure is to cultivate culture in beds, an organization of space suitable for flood irrigation.
The presence of water in surface or underground is necessary, but not sufficient to create such oasian spaces. The Sahara is a typical example of the actualization of this potentiality, but is not unique. Throughout the world, the oasis system feeds at least ten million people. However, there is no oasis in some desert regions, despite of the presence of water (the Kalahari, for example).
The creation of oases also has to do with historical contingencies: many oases have been created or developed to be stopover on trade routes (like the Saharan gold route or the Asian silk road).
Vincent Battesti, 1998