section_path: Numerical value, notation, and units
section_name: Numerical value, notation, and units
paragraph_text: Numerical value, notation, and units
  
  The speed of light in vacuum is usually denoted by a lowercase c, for "constant" or the Latin celeritas (meaning 'swiftness, celerity'). In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used c for a different constant that was later shown to equal √ 2 times the speed of light in vacuum. Historically, the symbol V was used as an alternative symbol for the speed of light, introduced by James Clerk Maxwell in 1865. In 1894, Paul Drude redefined c with its modern meaning. Einstein used V in his original German-language papers on special relativity in 1905, but in 1907 he switched to c, which by then had become the standard symbol for the speed of light.
  
  Sometimes c is used for the speed of waves in any material medium, and c for the speed of light in vacuum. This subscripted notation, which is endorsed in official SI literature, has the same form as related electromagnetic constants: namely, μ for the vacuum permeability or magnetic constant, ε for the vacuum permittivity or electric constant, and Z for the impedance of free space. This article uses c exclusively for the speed of light in vacuum.