The abrupt and unwanted variations of one or more signal characteristics, such as the interval between successive pulses, the amplitude of successive cycles, or the frequency or phase of successive cycles. Although widely used in fields such as telecommunications, the term jitter is seldom used in time and frequency metrology, since terms such as phase noise are more descriptive.

**Julian Day or Julian Date (JD)**

An integer day number obtained by counting days from the starting point of noon on 1 January 4713 B.C. (Julian Day zero). One way of telling what day it is with the least possible ambiguity. The Modified Julian Date (MJD) has a starting point of midnight on November 17, 1858. You can obtain the MJD by subtracting exactly 2 400 000.5 days from the JD.

A unit of frequency that represents one thousand cycles per second (10^{3} Hz).

Created May 12, 2010, Updated March 1, 2023