1. Change in frequency of sound or light waves caused by the relative motion of the source and the viewer: The Doppler effect refers to the perceived alteration in the frequency as a wave changes, like the distance between the source and the observer. For example, the sound of a siren on a moving vehicle appears to change as it approaches and passes a listener.

The Doppler effect is also the shift of spectral lines due to a body's motion toward or away from an observer.

Astronomers can tell by the Doppler effect if a distant star is moving toward or away from us.
2. Etymology: named after Christian J. Doppler (1803-1853), an Austrian physicist and mathematician who first described the principle known as the Doppler effect in 1842.

Doppler observed that the frequency of light and sound waves is affected by the relative motion of the source and the detector.

Another example is that of a train which approaches an observer, and a lower pitch after it passes by. The Doppler effect applies to all types of waves, including light.