Definition of the Doppler effect noun from the Oxford Advanced Learner's Dictionary

 

the Doppler effect

 noun
noun
BrE BrE//ˈdɒplər ɪfekt//
 
; NAmE NAmE//ˈdɑːplər ɪfekt//
 
[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 American Dictionary entry: the Doppler effect