The ITU Radiocommunication Assembly considering a) that there is a need for an adequate language with which to communicate the instability characteristics of standard frequency and time sources and measurement systems; b) that the classical variance does not converge for some of the kinds of random time and frequency instabilities; c) that major laboratories, observatories, industries and genera1 users have already adopted some of the Recommendations of the Sub-committee on Frequency Stability of the Technical Committee on Frequency and Time of the IEEE Society on Instrumentation and Measurement and the existence of the IEEE Standard No. 1139-1988 on “IEEE Standard Definitions of Physical Quantities for Fundamental Frequency and Time Metrology”.