In ecology, an oasis (; PL oases ) is a fertile area of a desert or semi-desert environment that sustains plant life and provides habitat for animals. Surface water may be present, or water may only be accessible from wells or underground channels created by humans. In geography, an oasis may be a current or past rest stop on a transportation route, or less-than-verdant location that nonetheless provides access to underground water through deep wells created and maintained by humans.

The word oasis came into English from Latin: oasis, from Ancient Greek: ὄασις, óasis, which in turn is a direct borrowing from Demotic Egyptian. The word for oasis in the latter-attested Coptic language (the descendant of Demotic Egyptian) is wahe or ouahe which means a "dwelling place". Oasis in Arabic is al-wāḥat ( واحه ).