Classic definition — Oppenheim: international law is "the body of rules which are legally binding on civilised states in their intercourse with each other" — the positivist, states-only definition (and its era's vocabulary). The modern working definition: international law is the body of legal rules and principles that governs the relations among states and between states and other subjects of international law — international organizations, individuals (in defined roles), and special entities — across war and peace, territory, sea, air, space, treaties, responsibility and human rights.
The World Court's functional frame: Article 38(1) of the ICJ Statute supplies the definition-by- application — the law the Court applies: treaties, custom, general principles of law, and the subsidiary sources (Unit 2's subject).