renamed, since the discussion has shifted.
NIST gives this definition:
"Primary Standard
"A standard that is designated or widely acknowledged as having the
highest metrological qualities and whose value is accepted without
reference to other standards of the same quantity. For example, NIST-F1
is recognized as a primary standard for time and frequency. A true
primary standard like NIST-F1 establishes maximum levels for the
frequency shifts caused by environmental factors. By summing or
combining the effects of these frequency shifts, it is possible to
estimate the uncertainty of a primary standard without comparing it to
other standards.
"In the time and frequency field, the term primary standard is
sometimes used to refer to any cesium oscillator, since the SI
definition of the second is based on the physical properties of the
cesium atom. The term primary standard is also commonly used, at least
in a local sense, to refer to the best standard available at a given
laboratory or facility."
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