The "Great Observatories" refer to a group of four major space telescopes launched by NASA to study the universe in different wavelengths of light. These observatories include the Hubble Space Telescope, which observes visible and ultraviolet light, the Chandra X-ray Observatory for X-rays, the Spitzer Space Telescope for infrared light, and the Compton Gamma Ray Observatory for gamma rays. Together, they provide a comprehensive view of astronomical phenomena. Each observatory has unique capabilities that allow scientists to explore various aspects of the cosmos, from the formation of stars to the behavior of black holes. By working in tandem, the Great Observatories have significantly advanced our understanding of the universe and its many mysteries.