A more thorough explanation:

Definition: Federal common law is a body of decisional law derived from federal courts when adjudicating federal questions and other matters of federal concern, such as disputes between the states and foreign relations, but excluding all cases governed by state law.

For example, nonstatutory law applying to interstate streams of commerce is an example of federal common law. This means that federal courts can create laws based on their decisions in cases that involve federal issues, even if there is no specific statute or constitution that addresses the issue.

Before the landmark case of Erie v. Tompkins in 1938, there was also a general federal common law that applied to all disputes heard in federal court. However, since Erie, federal courts are bound to apply the substantive law of the state in which they sit. Therefore, there is no longer a general federal common law applicable to all disputes heard in federal court.

In summary, federal common law is a body of law created by federal courts when adjudicating federal questions and other matters of federal concern, excluding cases governed by state law.