From the Wikipedia page [1]
In present-day Spain a mancomunidad (Aragonese: mancomunidat, Asturian: mancomunidá, Catalan: mancomunitat, Basque: mankomunitatea; in English "commonwealth") is a free association or commonwealth of municipalities. A mancomunidad is a legal personality, and can exist either for a particular period to achieve a concrete goal or can exist indefinitely.
A Spanish mancomunidad constitutes a local entity within the national legal framework, to which those municipalities delegate some of their functions and powers. It is similar to a comarca, with the difference that comarca has somewhat different meanings in the various autonomous communities of Spain and mancomunidad is defined identically throughout the country. The municipalities in a single mancomunidad need not be coterminous (though they usually are). They are required to set a clear goal, create management bodies distinct from those of the individual municipalities, and provide the mancomunidad with its own budget.