The Ultimate Quotable Einstein by Albert Einstein

Einstein is most famous for his theory of relativity.

In his first papers he referred to it as the "relativity principle. " The term "theory of relativity" was first used by Max Planck in 1906 to describe the Lorentz-Einstein equations of motion for the electron, and it was finally adopted by Einstein in 1907 in his reply to an article by Paul Ehrenfest, who had also used Planck's term. But Einstein continued to use "relativity principle" in the titles of articles for several years, since a "principle" is not a "theory" but something that is borne in mind when formulating a theory. In 1915, Einstein began to refer to the 1905 theory, having to do with space and time, as the "special theory," to distinguish it from his new theory of gravitation, the "general theory. " See Stachel et al., Einstein's Miraculous Year, 101–102; and Fölsing, Albert Einstein, 208–210