Semicircular window divided into three lights (compartments) by two vertical mullions, with the central light usually wider than the two side lights.
Its name comes from its use in the Baths of Diocletian in Rome (AD 302). It was revived in the 16th century by Andrea Palladio and others in the form of a window having an arched central light flanked by narrower, square-headed apertures, known as a Palladian or Venetian window.