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The degree to which an oscillating signal produces the same frequency for a specified interval of time. It is important to note the time interval—some devices have good short-term stability while others have good long-term stability. Stability does not determine whether the frequency of a signal is right or wrong. It only indicates whether that frequency stays the same. The Allan deviation is the most common metric used to estimate frequency stability, but several similar statistics are also used.
NISTIR 8323 from NIST T&F Glossary