A little information about as coins (plural: asses) is here in this section of Sestertius.net; we have split them up between Roman Republic asses and Roman Empire asses. Note that an ancient Republican as may be more scarce, but not necessarily more expensive, than an imperial as.
The as was first produced in 280 BC, and around 221 BC became the standard denomination in Rome — other coins made after this date were usually named by their worth in asses. (For example, the word “semis” means “half”, and the coin called semis was worth half an as.) The coins were made until Diocletian’s coinage reforms in the 280s AD. Early asses showed the two-faced god Janus; later as coins showed the emperor’s bust.
See also: Aureus rare Roman coins and numismatic collectibles
“As” in other languages
In most languages that might be found in as for sale listings, the word is the same as English; an exception is Italian, where it is asse.