Unique
Unique. Only one known specimen extant. The concept of uniqueness is rare for coins, it is common for medals, as many medallic items are inscribed with a recipient's name; these are, in effect, unique, as there exists only one specimen with that exact lettering. A unique coin is extremely valuable because of its inherent scarcity; this is in contrast to many unique medals inherently scarce but not valuable (for, perhaps lack of demand). The only true claim to the status of being unique is for only one piece to have been made. Otherwise, a second or subsequent specimen may conceivably turn up.
excerpted with permission from
An Encyclopedia of Coin and Medal Technology
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON
Roger W. Burdette, Editor