What is called an oasis? Why do oases appear in the desert in the desert?

Oases are islands with green vegetation, which are surrounded on all sides by desert. Oases form in places where groundwater is close enough to the surface and plant roots can reach the water. In other cases, water breaks through on its own to the surface, forming a small pond around which trees grow. Oases are places where animals can access drinking water in the dry desert. Also, many oases are human habitats.