Subsidiarity

Etymology

From Latin subsidiarius ("of a reserve, serving as support"), from subsidium, which meant the reserve troops held in support behind the front line. The buried image is telling: the higher authority is meant to act like reserves — held back, committed only when the local level cannot cope. The principle was framed in Catholic social teaching and later became a cornerstone of European Union law.

noun · /sub-sih-dee-AIR-ih-tee/
The principle that matters should be handled by the smallest or lowest competent authority, with higher levels stepping in only when necessary.
What it names
It offers a rule for where decisions belong, resisting the natural drift of power upward toward the center.
Example
Under subsidiarity, the union set only broad standards and left the details to each member state.
Related idea
"Federalism" divides power between central and regional governments by fixed constitutional rule; subsidiarity is the guiding principle for how low that division should push authority. One is a structure, the other a criterion for filling it.