What is Ayuntamiento?

Legal Definition
Ayuntamiento (Spanish pronunciation[aʝuntaˈmjento]) is the general term for the council of a municipality, or sometimes the municipality itself, in Spain and Latin America. Historically "ayuntamiento" was often preceded by the word excelentísimo (English: "most excellent"), when referring to the council. This phrase is often abbreviated "Exc.mo Ay.to"

In Catalan-speaking parts of Spain, municipalities generally use the Catalan cognate, ajuntament, while Galician ones use the word concello, Astur-Leonese conceyu and Basque udaletxea. Ayuntamiento is mainly used in Spain; in Latin America alcaldía is also used to describe municipal governing bodies, especially the executive ones, where the legislative body and an executive one are two separate entities.

In Latin America several terms exist for the legislative bodies of municipalities. The term consejo is used in Argentina, Chile, Colombia, Costa Rica, and Peru. In Mexico the term ayuntamiento is used to describe the council. Puerto Rican municipalities have a legislatura municipal. In Peru the term ayuntamiento is never used. It is Municipalidad or Consejo Provincial o Distrital executive functions in most of these countries is handled by an executive alcalde, the Mayor (not to be confused with the historic alcalde, who was a magistrate).

Since ayuntamiento is a metonym for the building in which the council meets, it also translates to "city/town hall" in English.
-- Wikipedia
Legal Definition
Spanish law. A congress of persons the municipal council of a city or town. 1 White's Coll. 416; 12 Pet. 442, notes.
-- Bouviers Law Dictionary
Legal Definition
In Spanish law. A congress of persons; the municipal council of a city or town. 1 White, Co.ll. 416; Friedman v. Goodwin, 9 Fed. Cas. 818.
-- Black's Law Dictionary
Legal Definition
A Spanish municipal council.
-- Ballentine's Law Dictionary