不需空氣或氧氣生長或維持生命的有機體，甚至可能死於有氧環境中。專性厭氧生物只能生長於無氧環境中。兼性厭氧生物能生長於有氧或無氧環境中。微好氧生物則是在少量氧氣的環境中生長狀態最佳。此詞彙特別用指微生物。Organisms that do not require air or free oxygen for growth or maintaining life, and may even die in its presence. Those that grow only in the absence of oxygen are obligate, or strict, anaerobes. Some species, called facultative anaerobes, are able to grow either with or without free oxygen. Certain others, able to grow best in the presence of low amounts of oxygen, are called microaerophiles. Used especially in reference to microorganisms.