Federalism
Etymology
From Latin foedus, “covenant, treaty, league,” from the same root as “fidelity” and “faith.” The idea is a binding pact among parties. The term crystallized in debates over the United States Constitution in the 1780s, and now names any arrangement that constitutionally splits sovereignty between a center and its parts, from federations to looser unions.
noun · /FED-ur-uh-liz-um/
A system of government in which power is divided between a central authority and constituent regional units, each with its own sphere.
What it names
The structural answer to “who decides what” in large or diverse countries — the reason the same issue can be national in one place and local in another.
Example
Under the country’s federalism, education was a regional matter, so the law changed the moment you crossed a state line.
Related idea
Subsidiarity is the related principle that decisions should be made at the most local capable level; federalism is the constitutional structure that can put such a principle into practice.