1. Of an environment: one in which air (oxygen) is absent. In the case of a depositional environment, one with 0.0–0.1 ml of dissolved oxygen per litre of water. Compare aerobic; dysaerobic.

2. Of an organism: one able to grow only in the absence of oxygen, i.e. an anaerobe.

3. Of a process: one that can occur only in the absence of oxygen.