The description of the word you requested from the astronomical dictionary is given below.
Between 1900 and 1920, Albert Einstein (1879 - 1955) explained a number of imperfections in the classical theories that were used at the time. He explained Brownian motion of dust particles, which happens because of collisions with invisible molecules of air. Einstein showed that the emission and absorption of light are quantum effects, and he was therefore one of the founders of quantum theory. In 1905 Einstein published the Special Theory of Relativity, which indicates that space and time are not wholly separate but together form a single space-time continuum, and that mass and energy are equivalent (the famous formula (E = m c^2)). In 1915 Einstein published the General Theory of Relativity, which states that gravity comes from the curvature of space-time because of the presence of mass. All of these theories have been tested often and in great detail and have been shown to be correct so far. Because of this, Einstein's fame is deservedly great.