- The speed at which a wave propagates through a given medium. It is the speed at which given points within it, such as crests and troughs, travel. In a vacuum it is equal to the product of the frequency and wavelength. In a dispersive medium the propagation speed of electromagnetic waves varies as a function of the frequency, while in a non-dispersive medium the propagation speed is independent of the frequency. Also called wave speed, wave celerity, or celerity.