Why is it even important?

The words plaza and piazza have the same Greek origin, with plaza being the Spanish adaption and piazza the Italian one. However, the U.S. has adopted the Spanish 'plaza' to define its urban public squares, which have pretty much become auto-oriented models. For instance, the New Urbanism Lexicon, the de facto dictionary for progressive urban planning, defines a plaza as:

A public space at the intersection of important streets set aside for civic purposes and commercial activities. A plaza is circumscribed by frontages; its landscape consists of durable pavement for parking and trees requiring little maintenance. All parking lots on frontages should be designed as plazas with the paving not marked or detailed as parking lots.

That's three references to parking in one definition. It's no wonder the term shopping plaza doesn't elicit elegant examples of memorable placemaking.

The definition of a piazza however, still retains its definition as an open public square, especially in Italian towns, surrounded by buildings, and usually the center of public life. In other words, it's still defined by its historic heritage as a pedestrian-oriented, often pedestrian-only destination, designed to be the heart of a neighborhood or town, filled with cafes, restaurants and bars and host to a series of local events and public markets. See a more detailed description in "What emerging generations really want: A piazza… 2011".

So, how does one technically distinguish a piazza from a plaza? For that we turn to Moule & Polyzoides, where you'd be hard pressed to find an urban design firm in the U.S. that is more knowledgeable about piazzas and plazas. It shouldn't be a surprise that co-founder Stefanos Polyzoides is from Greece, the birthplace of the forum and agora, and a direct precursor to the piazza and public market. Based on their criteria, here's how to tell a piazza (which they also refer to as a Mediterranean plaza) from an Americanized plaza:

1. Strong Enclosure - Piazzas feel like being in human-scaled outdoor rooms; very large courtyards, not like the aforementioned parking, cars and sometimes skyscrapers that are associated with plazas.
2. Wall Continuity - Piazzas are almost always surrounded on all sides by buildings with pedestrian-sized passageways, as opposed to the minimum two-lane traffic-filled streets that break up the sense of place in plazas.
3. Continuous Accessible Ground Floor - Piazzas are true to their Greek origin, acting as open stages by nature, deemphasizing landscaping features other than its floor. Plazas are often completely filled by fountains, monuments and other over-designed landscaping elements (see Georgetown Waterfront in Washington DC where the entire plaza is a fountain).
4. Open to Multiple Use/Non-Specific Design - The buildings on piazzas are designed to be timeless, so that as the economy and culture changes, so do the uses in the building… seamlessly. For some reason, many plazas seem to insist on featuring large buildings that are easily dated.
5. Temporariness vs. Permanence - As with a stage, the blank slate that is a piazza leaves the imagination to whoever is planning an event in it that day, unlike many plazas that are designed for specific experiences, from a fountain show to a landscaped viewing garden to picnic areas… which are fine for a park.
6. Day and Night - A piazza is in full use day and night by the locals - it can be a playground in the morning, welcome a concert in the afternoon, be a dining scene in the evening, and host a movie later that night. Plazas commonly have dead zones during certain times of the day if it's not positioned as a major tourist attraction.
7. Design of the Threshold between Public and Private Spaces This is a big one. Piazzas emphasize the private lingering spaces within it, from an outdoor dining area to a local coffeehouse or pub on the corner, it's a third place filled with third places. Plazas tend to feature more corporate or large public venues.
8. Cars Matter at Times - Piazzas can accommodate cars when necessary, such as when needed for special occasions, versus plazas where accommodating cars is necessary, period.
9. People-Watching as Sport - This is what makes a piazza a piazza… as a stage, there is always a spontaneous human activity show going on. In other words, life is never dull in a piazza.