A change in the observed frequency of a wave that occurs when the source and the observer are moving relative to each other.
The variation of the apparent frequency of waves, such as sound waves, with change in distance between the source and the receiver. The frequency seems to increase as the distance decreases and to decrease as the distance increases.
When a source of sound is moving quickly relative to an observer, it causes a shift in the frequency of the sound waves reaching the observer. If the source is approaching, the sound appears to have a higher pitch (frequency) compared to when the source is moving away. This happens because the wavelengths of sounds from an approaching source become shorter and compressed, while those from a receding source become longer and stretched.
Various medical ultrasound scanning techniques make use of the Doppler effect.