Term for a single surviving copy of a printed text

I know that there is a precise word which refers to a single surviving copy of a printed text, but I can't recall it or find it anywhere in my notes. Help?

Unica / Unicum

Google Definition: a unique example or specimen.

Example use: “The concept of a unicum is difficult for the average library user to understand, since books, by their very nature, exist in more than one copy. That’s the genius of Gutenberg’s invention, after all,”

This quote is from an article explaining a University of Illinois project to scan books that exist in only one copy. The article is here

The university project is called Project Unica

The German term is "das Unikat" (not limited to books), and my dictionary tells me the translation is the old and slightly obsolete english noun "unique".

https://en.oxforddictionaries.com/definition/unique

Chambers 21st Century Dictionary says: "anything, especially formerly a coin or medal, of which there is only one copy"