SpO2 is the saturation of oxygen in the blood as measured by a pulse oximeter. A pulse oximeter is non-invasive and measures the SpO2 through the capillary bed, often at a fingertip. This is different from SaO2, which measures the oxygen saturation of arterial blood and requires blood sampling.
The "O2" in SpO2 stands for the two atoms of oxygen that make up an oxygen molecule. The "S" stands for the saturation of hemoglobin in the blood, or the percentage of hemoglobin, which carries oxygen. The "p" stands for the pulse, which differentiates SpO2 from the SaO2 laboratory blood test.