This denomination means 'unity' and was a standard unit in roman coinages when first struck. It was first minted at around c. 280 BC. At the beginnin gof the Roman Empire it was struck in pure copper which gave its yellowish appearance. By the end of the 3rd century AD it was made of Bronze.
An identifying mark on an As coin is the emperor who is always shown wearing a laurel wreath.
Ancient Roman As Coins for Sale