regions in deserts and semideserts with intensive agriculture on irrigated land or tracts with trees, shrubs, and herbaceous vegetation. They occur wherever an area receives more water than the surrounding regions. Natural watering is provided by fresh groundwater lying close to the surface, springs, periodic flooding, and the overflow of rivers, and artificial watering is made possible by irrigating with water from rivers, lakes, canals, and wells. Oases vary in size from dozens of hectares to tens of thousands of square kilometers (for instance, the Nile Valley in Africa, the lower reaches of the Amu Darya, and the Fergana Valley in Middle Asia). Oases are populated areas in deserts where food and industrial crops, especially cotton, are cultivated.

Ice-free areas in Antarctica are also called oases.