liquid junction

Any junction between two electrolyte solutions of different composition. Across such a junction there arises a potential difference, called the liquid junction potential. In the operational pH cell the junction is between the test, or pH standard, solution and the filling solution or the bridge solution of the reference electrode.
PAC, 1985, 57, 531 (Definition of pH scales, standard reference values, measurement of pH and related terminology (Recommendations 1984)) on page 541