Why is the symbol for the speed of light the letter "c"? The International Standards Organization (ISO) recommended letter symbol for the velocity of sound, and for the velocity of wave propagation, is also c. Please also note that the IEC recommended symbol for the velocity (speed) of electromagnetic waves in vacuum is c_0 (c subzero) and this will eventually be adopted by ISO. Anders Thor, Chairman of ISO technical Committee comments on why the letter c is used as the symbol for the quantity speed of light and of all electromagnetic waves in vacuum: "The symbol for this quantity is given in the International Standard ISO 31, Quantities and Units, as c (italic) or c (italic) sub 0. ISO did not invent these symbols. They were used in physics and technology long before ISO was founded in 1947. ISO only implemented what was already well established practice in this field. I am not sure how the symbol c was first introduced in physics. The only thing I could think of is that it is derived from the French term "celerite," which means phase velocity. You could find this term in the French edition of ISO 31, Part 2." We may have to ask Einstein after all.