Doppler effect

n. apparent change in the frequency of sound or light waves due to a change in the distance between the source of the waves and the receiver

The Doppler effect, named after Christian Doppler, is the change in frequency and wavelength of a wave as perceived by an observer moving relative to the source of the waves. For waves that propagate in a wave medium, such as sound waves, the velocity of the observer and of the source are reckoned relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from either motion of the source or motion of the observer. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in special relativity, only the relative difference in velocity between the observer and the source needs to be considered.
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Doppler effect n. An apparent change in the frequency of waves, as of sound or light, occurring when the source and observer are in motion relative [more]Doppler effect - Community and Resources

Noun
1. change in the apparent frequency of a wave as observer and source move toward or away from each other
(synonym) Doppler shift
(hypernym) propagation
(hyponym) red shift

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.