The word oasis comes into English via Greek ὄασις oasis, borrowed directly from Egyptian wḥ3t or Demotic wḥỉ. It was not borrowed from Coptic ouaḥe (*/waħe/), as is sometimes suggested; the Greek word is attested several centuries before Coptic existed as a written language. Its presence in English, stems from the time when English speakers were first becoming world travellers. In Samuel Purchas's 1613 travel book, he first mentions of oasis as being a 'fertile place' in the Libyan desert.
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.
The location of oases has been of critical importance for trade and transportation routes in desert areas. Caravans must travel via oases so that supplies of water and food can be replenished. Thus, political or military control of an oasis has in many cases meant control of trade on a particular route. For example, the oases of Awjila, Ghadames and Kufra, situated in modern-day Libya, have at various times been vital to both North-South and East-West trade in the Sahara desert.