The term Specimen is used to describe coins with a variety of finishes that are distinct from the appearance of circulation issues but do not fit any of the Proof categories. This can include early US coins with bold strikes, very brilliant fields and semi-frosted devices as well as modern US coins that were produced with matte or other unusual finishes.
An example is this unique 1873 Closed 3 Five Dollar gold piece that was certified by NGC as a Specimen strike. It is the only such piece certified by either PCGS or NGC. It has the visual characteristics of a highly contrasted cameo proof, yet was struck from circulation dies.
This unique 1873 $5 Closed 3 Specimen strike has an experimental finish and is a hybrid between a proof and a mint state coin. This specimen strike was from specially prepared mint state dies that created a specimen with the appearance of a proof.