I'm trying to remember a word that is used to describe a perfect specimen or sample of something. That is, a unit that best represents other units.
I think this term may come from stamp collecting. My mind associates this word with words like supernumerary, nomenclature, and nominal. It may even sound like one of those words.
The intended usage is something along these lines, "An uncompressed XXXXXXXXX file was then transcoded to numerous candidate codecs and objective measurements were made between the XXXXXXXXX and derivative files." In this example, XXXXXXXXX would be replaced by the word I'm trying to think of.
Many thanks in advance for any assistance you can offer!