It is the change in the wavelength (and frequency) of a wave as a result of the motion of either the source or receiver of the waves.

If the source and the receiver are approaching each other, the frequency of the wave will increase and the wavelength will be shortened- sounds will be higher in pitch and light will be bluer. If the source and receiver are moving apart, sounds will become lower-pitched, and light will appear redder.

Astronomers analyze Doppler shifts of light and radio waves to measure the velocities and distances of remote objects.