reference electrode

https://dx.doi.org/10.1351/goldbook.R05229
  1. An external electrode system which comprises an inner element, usually mercury–mercury(I) chloride (calomel), silver–silver chloride or thallium amalgam–thallium(I) chloride, a chamber containing the appropriate filling solution and a device for forming a liquid junction, e.g. capillary, ceramic plug, fritted disc or ground glass sleeve.
    Source:
  2. An electrode that maintains a virtually invariant potential under the conditions prevailing in an electrochemical measurement, and that serves to permit the observation, measurement, or control of the potential of the indicator (or test) or working electrode.
Source:
See also:
Orange Book, p. 65
PAC, 1994, 66, 2527 (Recommendations for nomenclature of ionselective electrodes (IUPAC Recommendations 1994)) on page 2533