Doppler effect noun
: a change in the frequency with which waves (as sound, light, or radio waves) from a given source reach an observer when the source and the observer are in motion with respect to each other so that the frequency increases or decreases according to the speed at which the distance is decreasing or increasing—compare shift a
Seen & Heard
What made you want to look up Doppler effect? Please tell us where you read or heard it (including the quote, if possible).