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 150 mmHg, 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 40 mmHg at the peak of mount Everest.
Human cells, however, aren't usually exposed to 150 mmHg of pO2. The pressure drops between different compartments in the body so that the tissues only have around 10 to 18 mmHg pO2. This means that if you're growing these cells as a culture outside of the body, then having them at a pO2 of 15 mmHg wouldn't cause them to become hypoxic. Hypoxia is all relative to the cell or organism and it's natural environment.
Although hypoxia may sound dangerous is very important in the formation of placenta, heart, bone and vasculature in embryos. This is done by using the Hypoxia Inducible Cascade (HIF) signalling cascade.