Quite often, when I see a house that I love, it has a Palladian window. Palladian windows are named for Andrea Palladio (1508-1580), the Italian Renaissance architect who invented them in the sixteenth century. They are identified by the arch on top and the narrow panels on both sides of the central window. The Palladian window is one of Palladio's most popular and widely imitated design motifs. So appealing and influential was the design of Palladio that his work continues to inspire architects to this day.
Palladianism became popular in Europe in the 17th and 18th centuries, and experienced a surge of popularity in North America later in the late 18th century. Most notably, Thomas Jefferson promoted Palladianism in many of his architectural designs. Jefferson's Monticello and the University of Virginia have many architectural features characteristic of Palladian style. In fact, Jefferson referred to Palladio's "I Quattro Libri dell'Architettura" as his 'bible'.
So pervasive was the Palladian influence, that features of Palladian style can be found in Beaux Arts Classicism, Colonial Revival, Classical Revival, Federal, Georgian Revival, Neoclassicism, Shingle, Queen Anne styles. However, with time, the term Palladianism has become largely misused to describe anything with classical motifs.
Architects usually put Palladian windows in front of staircases or atriums, in order to provide a lot of light to large halls. Historically, Palladian windows most often appear in public buildings and the homes of the wealthy because glass was expensive int he eighteenth century and Palladian windows require more glass than regular windows.