deviation of a glass electrode from the hydrogen-ion response function. An example often encountered is the error due to sodium ions at alkaline pH values, which by convention is regarded as positive.
Source:
PAC, 1985, 57, 531. 'Definition of pH scales, standard reference values, measurement of pH and related terminology (Recommendations 1984)' on page 540 (https://doi.org/10.1351/pac198557030531)