El Zócalo is a term used to refer to the main plaza of a Mexican town. It is believed that the word comes from the Italian term zoccolo, which means plinth or pedestal. In the 19th century, a pedestal was set up in the center of Mexico City's main square that was to be the base for a monument that would commemorate Mexican independence. The statue was never put in place and people began to refer to the square itself as Zócalo.
Now in many towns in Mexico the main square is called the Zócalo.
Colonial Town Planning
In 1573, King Philip II ordained in the Laws of the Indies that colonial towns in Mexico and other Spanish colonies should be planned in a certain manner. They were to be laid out in a grid pattern with a rectangular plaza in the center surrounded by straight streets that intersect at right angles. The church was located on one side (usually the east) of the plaza, and the government building was to be built on the opposite side. Buildings surrounding the plaza would have arcades to allow merchants to conveniently set up shop there. The central plaza was thus designed to be the religious, political, economic and cultural heart of the city.
Most of Mexico's colonial towns reflect this design, but there are some, such as the mining towns of Taxco and Guanajuato, which were built on locations with uneven topography where this plan could not be fully implemented.
The Mexico City Zócalo
The Mexico City Zocalo is the original and most famous one. Its official name is Plaza de la Constitución. It is located over the ruins of the Aztec capital city Tenochtitlan, and the main temple of the Aztecs can be seen in the Templo Mayor archaeological site located just to the northeast of the plaza, beside the cathedral.
Throughout its history the plaza has gone through many incarnations; now it is a huge paved surface of 830 by 500 feet (195 x 240 meters) with just a large flag in the center. The space is used for concerts, events and sometimes protests.
By Any Other Name...
The term Zócalo is common, but some cities in Mexico use other words to refer to their main square. In San Miguel de Allende, the main square is usually referred to as El Jardín and in Mérida it's called La Plaza Grande. When in doubt you can ask for "la plaza principal" or "plaza mayor" and everyone will know what you are talking about.