What does the term asymmetry potential mean

For an ideal electrode the potential difference between the reference and measuring electrode is 0mV at pH 7. As the ideal electrode doesn’t exist the deviation of a few mV at pH 7 that is found with real electrodes is called the asymmetry potential, Eo. In practical terms it is a measure of the performance of the reference electrode – if the result is outside of +/- 15mV this is indicative of problems with the reference electrode such as contaminated electrolyte or reference element poisoning.