Doppler Effect

apparent change in frequency of a wave due to the relative motion of the source and observer. For example, when they move apart the wavelength is stretched making the frequency lower.

The phenomenon evidenced by the change in the observed frequency of a sound or radio wave caused by a time rate of change in the effective length of the path of travel between the source and the point of observation.

The change in the observed frequency (or wavelength) of a wave, caused by a time rate of change in the effective path length between the source and the point of observation.