the shift in frequency of acoustic or electromagnetic radiation emitted by a source moving relative to an observer a s perceived by the observer: the shift is to higher frequencies when the source approaches and to lower frequencies when it recedes. For example, when one hears the sound of the siren on a police car, the pitch is perceived to be going higher as the car approaches and going lower as the car recedes. Named after C.J. Doppler (1803 - 53), Austrian physicist. —Handbook for Preclears Glossary (app 9.7.90)