The Collaborative International Dictionary of English v.0.48:

Piazza \Pi*az"za\, n.; pl. Piazzas. [It., place, square,
   market place, L. platea street, courtyard. See Place.]
   An open square in a European town, especially an Italian
   town; hence (Arch.), an arcaded and roofed gallery; a
   portico. In the United States the word is popularly applied
   to a veranda.
   [1913 Webster]

         We walk by the obelisk, and meditate in piazzas. --Jer.
                                                  Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

piazza
    n 1: a public square with room for pedestrians; "they met at Elm
         Plaza"; "Grosvenor Place" [syn: plaza, place, piazza]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "piazza":
   agora, campo, forum, gallery, lanai, market, market cross,
   marketplace, mart, patio, place, plaza, porch, rialto,
   sleeping porch, solarium, square, stoop, sun porch, veranda