Final answer:
Gross Domestic Product (GDP) is the US economic indicator that is only available on a quarterly basis, as it is released by the Bureau of Economic Analysis (BEA) every three months with subsequent revisions.
Step-by-step explanation:
The important US economic indicator that is available only on a quarterly basis is the Gross Domestic Product (GDP). GDP is a measure of the total economic output of a country and is a crucial indicator of its economic health. It provides a broad indication of a society's standard of living, though it does not account for non-economic factors like leisure or environmental quality. Economic indicators such as GDP help to describe current economic activity but also help predict future economic trends. The Bureau of Economic Analysis (BEA) releases GDP estimates quarterly, followed by revisions and updates as more data becomes available. Other indicators like the Consumer Price Index (CPI), Purchasing Managers' Index (PMI), and nonfarm payrolls are released at different intervals.