A foreign country includes any territory under the sovereignty of a government other than that of the United States.

The term "foreign country" includes the country's airspace and territorial waters, but not international waters and the airspace above them.

The term "foreign country" does not include Antarctica nor does it include U.S. territories, uninhabited insular areas (including Johnston Island), and areas operating under a Compact of Free Association.

This information is found in Publication 54, Tax Guide for U.S. Citizens and Resident Aliens Abroad.