Definition of the Doppler effect noun from the Oxford Advanced American Dictionary

 

the Doppler effect

 noun
noun
NAmE//ˈdɑplər ɪˌfɛkt//
 
[singular](physics)
 
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the way that sound waves, light waves, etc. change according to the direction that the source is moving in with relation to the person who is observing
See the Oxford Advanced Learner's Dictionary entry: the Doppler effect