In Tuscany, panzanella is what inevitably happens to stale bread: cut it in chunks and make it into a salad with lots of tomatoes, cucumber, red onions and fresh basil, and perhaps anchovies, cheese and other things, too, with plenty of green gold olive oil.
The name comes from pane (bread) and zanella (a Tuscan term for the oval container in which the salad was served, even back in the 16th century, although back then it didn't have tomatoes).
The panzanella romana can be something similar, or else much the same as bruschetta or crostini, slices of bread or toast with tomatoes and oil, or something else, spread on. It can also mean a special flat bread, made for the purpose.
Monterotondo, just outside of Rome, holds an annual five day Sagra della Panzanella in early July.
Image by Jeremy Keith