[lip]

When potential is measured by using silver/silver chloride electrode, Ag+ would precipitate from the solution (as Ag).  This will increase the equilibrium [Cl-]( negligibly small increase ). So half cell potential of the electrode should change right  ? so in what sense we consider E of this electrode to remain to be constant and thereby use this as a reference  electrode ? or stability of reference electrode is just a relative term ?

[Borek]

Yes, technically the potential is not stable, but it is perfectly stable in practical terms. In typical case electrode (Ag wire covered with AgCl) is immersed in 1 M (or comparable concentration) solution of KCl. As long as the electrode is used only as a reference currents flowing through it are so low, changes in the concentration of Cl^- are completely negligible.

[lip]

Ok. Thanks Borek