Datum reference - Wikipedia
https://en.wikipedia.org:443/wiki/Datum_reference
WEBA datum reference or just datum (plural: datums) is some important part of an object—such as a point, line, plane, hole, set of holes, or pair of surfaces—that serves as a reference in defining the geometry of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the nominal value, which ...
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