The polarization index, or PI, is the ratio between two insulation resistance readings taken during the same test: the 10-minute value divided by the 1-minute value. Since both are taken on the same equipment at the same temperature, the ratio removes almost entirely the thermal dependence that makes absolute values hard to compare.
The principle is that in healthy insulation the absorption current decays over time and the resistance rises, whereas if there is moisture or dirt the leakage current dominates and the reading stays flat. A high PI means the curve is still climbing; one close to unity means it is not.
As a guide, below 1 is taken as unacceptable, 1 to 2 as questionable and above 2 as good, with caveats by insulation type: in modern very-high-resistance insulation the index loses meaning, and IEEE 43 itself warns that it should not be applied when the 1-minute reading exceeds 5,000 MΩ.