All the law—national, international, or mixed—that applies to all persons, businesses, and governments that perform or have influence across state lines.
Transnational law regulates actions or events that transcend national frontiers. It involves individuals, corporations, states, or other groups—not just the official relations between governments of states.
An almost infinite variety of transnational situations might arise, but there are rules or law bearing upon each. Since applicable legal rules might conflict with each other, "choice of law" is determined by rules of conflict of laws or private international law. The choice, usually between rules of different national laws, is made by a national court.
In other types of situations, the choice might be between a rule of national law and a rule of "public international law," in which case the choice is made by an international tribunal or some nonjudicial decision-maker, such as an appointed body.