An Etymological Dictionary of Astronomy and Astrophysics

Change in frequency of a wave (light, sound) due to the relative motion of source and receiver. Approaching objects have their wavelengths shortened. Receding objects have emitted wavelengths lengthened.

See also: Doppler, after Christian Andreas Doppler (1803-1853), Austrian physicist who first described how the observed frequency of sound and light waves is affected by the relative motion of the source and the detector; → effect.