Diocletian window

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.

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▪ architecture

also called  thermal window 

      semicircular window or opening divided into three compartments by two vertical mullions. Diocletian windows were named for those windows found in the Thermae, or Baths, of Diocletian (now the church of Santa Maria degli Angeli) in Rome. The variant name, thermal window, also comes from association with the Thermae. This type of window was used in the 16th century, especially by Andrea Palladio (Palladio, Andrea), and in the early 18th century by the English architect Richard Boyle, 3rd earl of Burlington (Burlington, Richard Boyle, 3rd earl of), one of the originators of the English Palladian style, and his followers.

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