- Deficiency in the amount of oxygen reaching body tissues.
- Depletion of dissolved oxygen in aquatic environments to levels that are detrimental or fatal to aerobic organisms, often caused by eutrophication.
- A condition in which <xref>tissues</xref> (especially the <xref>blood</xref>) are deprived of an adequate supply of <xref>oxygen</xref>; <xref>anoxia</xref>
- A reduced concentration of dissolved oxygen in an <xref>aquatic</xref> environment.
- oxygen deficiency causing a very strong drive to correct the deficiency