This is harder, but basically a dome is an arch turned around and around. People did not get this idea until the time of the Roman Empire, around 100 AD. A famous early example of a dome is Hadrian's Pantheon in Rome. Another great dome from the ancient world is the one in Hagia Sophia, in Constantinople. Architects built Hagia Sophia under the Emperor Justinian in the 500s AD.
About 1100 AD, after a big gap, architects in Europe began to build domes again. One example is the baptistry of Pisa. A really big dome was the later Duomo of Florence (1418).
Eyewitness: Building, by Philip Wilkinson, Dave King, and Geoff Dann (2000). Lavishly illustrated, like other Eyewitness books for kids, and with good explanations of most architectural terms.
Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University
Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.