A limes (or the Limes Romanus) was a border defense or delimiting system of Ancient Rome. It marked the boundaries of the Roman Empire.
The Latin noun limes had a number of different meanings: a path or balk delimiting fields, a boundary line or marker, any road or path, any channel, such as a stream channel, or any distinction or difference. In Latin, the plural form of limes is limites. The word limes, hence, was utilized by Latin writers to denote a marked or fortified frontier. This latter sense has been adapted and extended by modern historians concerned with the frontiers of the Roman Empire; e.g., Hadrian 's wall in north England is sometimes styled the Limes Britannicus, the frontier of the Roman province of Arabia facing the desert is called the Limes Arabicus, and so forth." (Wikipedia)
See: Limes denarius