Hypoxemia (or hypoxaemia) is generally defined as decreased partial pressure of oxygen in blood, sometimes specifically as less than 60 mmHg (8.0 kPa) or causing hemoglobin oxygen saturation of less than 90%.
Distinction from anemia and hypoxia
The hypoxemia definition as decreased partial pressure of oxygen excludes decreased oxygen content caused by anemia (decreased content of oxygen binding protein hemoglobin) or other primary hemoglobin deficiency, because they don't decrease the partial pressure of oxygen in blood.
Still, some simply define it as insufficient oxygenation or total oxygen content of (arterial) blood, which, without further specification, would include both concentration of dissolved oxygen and oxygen bound to hemoglobin. Inclusion of the latter would include anemia as a possible cause of hypoxemia (which, however, is not the case generally).
Hypoxemia is different from hypoxia, which is an abnormally low oxygen availability to the body or an individual tissue or organ. Still, hypoxia can be caused by hypoxemia, and such hypoxia is referred to as hypoxemic hypoxia, which is distinguished from e.g. anemic hypoxia. Because of the frequent incorrect use of hypoxemia, this is sometimes erroneously stated as hypoxic hypoxia.