The Hubble diagram, also known as the Hubble-Lemaître law, refers to a plot that shows the relationship between the distance of galaxies from Earth and their velocity of recession. It was discovered by Edwin Hubble in the 1920s and is a cornerstone of modern cosmology. The Hubble diagram essentially demonstrates that galaxies are moving away from each other at a velocity that is directly proportional to their distance from us, providing evidence for the expansion of the universe. This relationship forms the basis for the Hubble constant, which is a key parameter in determining the age, size, and future fate of the universe. Studies of the Hubble diagram continue to refine our understanding of cosmic expansion and the structure of the universe.