Given by \[\chi ^{\beta} = \mathit{\Phi }^{\beta} - \psi ^{\beta}\] where ψ β and Φ β are the outer and inner electric potentials of phase β, respectively.
Sources:
Green Book, 2nd ed., p. 59 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1974, 37, 499. 'Electrochemical nomenclature' on page 506 (https://doi.org/10.1351/pac197437040499)