main plaza of Madrid, Spain. It was reputedly named for a gate (puerta) that stood there until 1510 and had on its front a representation of the sun (sol). Throughout Madrid's history the square has been the focal point of transportation and of intellectual and economic activity. It was the first part of the city to be equipped with modern conveniences (electric lights, streetcars) and is the site of a New Year's celebration similar to that of St. Mark's Square, Venice.