Propagation speed is the rate at which a signal passes along a wire or cable (or is propagated across it) in a network. Propagation speed is also referred to as velocity factor or velocity of propagation; velocity is perhaps the better term because it refers to the specific definition of rate. The propagation speed or velocity of propagation is the speed of light (or what that speed would be in a vacuum) divided by the square root of the cable’s dielectric constant (the calculation of electrical energy it stores). This can be visualized as , where DC is the dielectric constant of whatever material the cable is. The dielectric constant of a material indicates its ability to store electrical energy. The propagation speed is expressed as a percentage in what way it is relative to the speed of light, which is the ideal propagation speed and occurs in a vacuum.
Propagation delay differs from propagation speed in that it describes the amount of time that the signal takes to pass along the cable (the slight delay that occurs due to the speed of light), rather than the rate.