Foreign law is the national law of a country other than the United States; the foreign law of a particular nation state defines the relationships within its borders.  The important primary sources of foreign law will depend on the type of legal system, but will often include the constitutions, statutes, regulations, and court decisions of the particular country you’re researching.

This differs from comparative law, which is the study of similarities and differences between two or more countries or legal systems, and international law, which deals with the norms/rules governing relations between two or more nations.