A golden unit or golden device is an ideal example of a device (such as a unit of measure) against which all later devices are tested and judged. The term "golden" is used to describe the precision of the device to standard specifications.
For example, in Paris there is a measure for the official "kilogram" unit of measurement. This is stored in a climate-controlled environment to prevent deviations.
References
Golden unit Wikipedia(Text) CC BY-SA