The Zócalo is the main plaza or square in the heart of the historic center of Mexico City. The plaza used to be known simply as the "Main Square" or "Arms Square," and today its formal name is "Constitution Square" (Plaza de la Constitución). This name does not come from any of the Mexican constitutions that have governed the country but rather from the Cádiz Constitution which was signed in Spain in 1812. However, it is almost always called the "Zócalo" today. This word literally means "base" or "plinth". Plans were made to erect a column as a monument to Independence, but only the base, or zócalo, was ever built. The plinth was destroyed long ago but the name has lived on. Many other Mexican towns and cities, such as Oaxaca and Guadalajara, have adopted the word "zócalo" to refer to their main plazas, but not all.
It has been a gathering place for Mexicans since Aztec times, having been the site of Mexica ceremonies, the swearing in of viceroys, royal proclamations, military parades, Independence ceremonies and modern religious events such as the festivals of Holy Week and Corpus Christi. It has received foreign heads of state and is the main venue for both national celebration and national protest.
Source: http://en.wikipedia.org/wiki/Zócalo
