Hypoxia

A cell or organism can be described as being hypoxic if they it is in an environment that has a lower pO₂ than the environment the cells are usually exposed to. At sea level the pO₂ is 150mmHg, so this is the usual level of Oxygen for a human, so falling below that level, by climbing a mountain for example, will cause hypoxia. This is because the pO₂ drops the further above sea level you get, dropping to about 40mmHg at the peak of mount Everest.

Human cells, however, aren't usually exposed to 150mmHg of pO₂. The pressure drops between different compartments in the body so that the tissues only have around 10 to 18mmHg pO₂. This means that if you're growing these cells as a culture outside of the body, then having them at a pO₂ of 15mmHg wouldn't cause them to become hypoxic. Hypoxia is all relative to the cell or organism and it's natural environment.