United States Generally Accepted Accounting Principles.
Sometimes abbreviated to "GAAP", especially in the United States.
Compared with International Financial Reporting Standards (IFRS), US GAAP is generally considered to be a relatively more rules based system, compared with the relatively more principles based approach of the IFRS regime.
However, even IFRS comprises a large number of detailed accounting rules.
Over time, differences between US GAAP and IFRS are being reduced.