Doppler effect (C.J. Doppler)

What is Doppler effect (C.J. Doppler)?

Doppler effect (C.J. Doppler) definition and meaning on Science terms:
The apparent change in wavelength of sound or light caused by the motion of the source, observer or both. Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. It occurs both in sound and light. How much thefrequency changes depends on how fast the object is moving toward or away from the receiver. Compare cosmological redshift.