Fundamental Speed

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The fundamental speed (usually denoted by the algebraic symbol c) is a term used to denote the relativistic properties of the speed of light. It refers to its unique characteristics of "invariance" (it has the same value in all [inertial] frames), and to its limiting character (a material body can approach, but not attain it; only free energy matches it).

The latter property is a virtual consequence of the former.

Sir Arthur Eddington used the labels "universal velocity" and "fundamental velocity" (the former referring to c's invariance). But "fundamental speed" is more apt, because the characteristics do not apply to the quantity's direction, only to its scalar magnitude.

João Magueijo, for example, uses the term in variable light speed theory.