The term ancient coins generally refers to coins struck before 1000 A.D., plus coins of the Byzantine Empire struck until its fall in 1453. All coins struck posterior to those described above as ancient are refered to as modern. This definition, although perhaps the most widely accepted, is not universally accepted.
Dictionary of Roman Coins
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ANCIENT COINS.--By the term ancient are meant all coins preceding the 9th century or the age of Charlemagne; and by modern all posterior to that period. (Pinkerton.)
The most ancient coins of the Romans are those cast with image of the ox, the sow, and the sheep; the double-headed Janus, the rostrum or beak of a ship, or the foremost half of a ship, ratis. Hence the coin was called ratitus.--Rasche.