1. adj. [Geology]

The condition of an environment in which free oxygen is lacking or absent.

Synonyms: anoxic

2. adj. [Geology]

A description of organisms that can survive in the absence of oxygen, particularly bacteria.

3. adj. [Drilling Fluids]

Pertaining to systems, reactions, or life processes of species, such as bacteria, in which atmospheric oxygen is not present or not required for survival.

Antonyms: aerobic

See: bactericide,  biodegradation,  sour gas,  sulfate-reducing bacteria