Obverse of a coin is the side which bears the more important device or inscription; the other side is called the reverse. In the United States the side bearing the date is called the obverse, irrespective of device. On ancient coins the side bearing the portrait of a ruler or the picture of a deity is always the obverse. Some writers claim that in classifying ancient coins the obverse must always be regarded as the side which received its impression from the lower die, i.e., the die supported by the anvil.
Source: Frey's Dictionary (American Journal of Numismatics, Vol. 50, 1916)