federacio

A federation: a body whose members are themselves bodies. Political ones, where states keep their own laws under a common government, and voluntary ones, where clubs or national associations band together and keep their independence. The adjective is federacia, federacio is the arrangement and the institution alike, and federi is to bring parties into one.

The Esperanto movement is full of them, which makes the word easy to place. The Internacia Fervojista Esperanto-Federacio, founded in 1948 for railway workers, has been affiliated to UEA since 1953 and still holds its own annual congress; the Italian speakers organise as the Itala Esperanto-Federacio. UEA itself is an asocio, not a federacio, though its members are national associations and it behaves like one. Which is the useful lesson: the label a body chooses is a matter of tradition, not of grammar.

One false seam to close, the same as on konstitucio. The -acio is not an Esperanto suffix. Federaci- is a single borrowed root, and there is no federi sitting underneath it that you built the noun from; the verb federi exists separately and shares the root, but you cannot manufacture -acio nouns at will the way you can manufacture -ado or -ejo ones. Civilizacio, konstitucio, federacio, organizacio all came in whole from the international vocabulary. Compare asocio, unio and organizaĵo when you need to name a body of people; only the last of those is genuinely assembled from Esperanto parts.