Dictionary.net

• The small space on a medal or coin beneath the base line of the subject intended for the date or other inscription.

1914 - Nuttall's Standard dictionary of the English language
By Nuttall, P.Austin.

• The small space on the face of a medal or coin left for a date, name, &c.-usually beneath the base-line of the subject engraved.

1874 - Etymological and pronouncing dictionary of the English language
By Stormonth, James, Phelp, P. H.