1580s, "open public square in an Italian town," from Italian piazza, from Latin platea "courtyard, broad street," from Greek plateia (hodos) "broad (street)," from platys "broad, flat" (from PIE root *plat- "to spread"). According to OED, mistakenly applied in English 1640s to the colonnade of Covent Garden, designed by Inigo Jones, rather than to the marketplace itself; hence "the veranda of a house" (1724, chiefly American English).