An oculus is a circular opening, especially one at the top of a dome that lets in light and air. The most famous example is the oculus at the center of the Pantheon in Rome, a massive temple built nearly 2,000 years ago. This opening, about 30 feet wide, is the building's only source of natural light. Rain falls through it onto the marble floor below, and on sunny days, a dramatic beam of light moves across the interior as the sun travels across the sky.

Architects use oculi (the plural form) to create striking visual effects and solve practical problems. Before electric lights, an oculus could illuminate a large space without requiring walls full of windows.

You might also encounter oculus in virtual reality, where Oculus is a brand name for VR headsets. The company chose this name because the headsets act like windows (or eyes) into digital worlds, connecting the ancient architectural meaning to modern technology.