Tax treaty definitions clarify residency and taxable persons, shaping applicability and interpretation under national tax laws. Article 3 defines territorial scope, key categories of persons and entities (including company, national, person, and enterprise of a Contracting State), and meanings of fiscal year and international traffic; it defines tax for treaty purposes excluding penalties. It further provides that undefined terms shall have the meaning given by the domestic law of the Contracting State applying the provision, with meanings under that State's tax laws prevailing.