Constitutional definitions clarify taxation, territorial and social categories, shaping legislative competence and tax scope. Article 366 assigns operative meanings to expressions used in the Constitution, fixing the scope of fiscal, territorial and social terms that determine legislative competence and tax incidence. It defines fiscal concepts such as agricultural income, corporation tax, taxation and tax on income, commercial terms including goods, services and goods and services tax, and financial instruments like borrow, debt and guarantee. It also defines categories and institutional terms-Scheduled Castes, Scheduled Tribes, socially and educationally backward classes, High Court, Part and Schedule-thereby channeling the substantive reach of other constitutional provisions and amended entries.