Meaning of Doppler effect

Dop'pler effect"

Pronunciation: (dop'lur), [key]
— Physics. Physics.
  1. (often l.c.) the shift in frequencyof acoustic or electromagnetic radiation emitted by a source moving relative to an observer as perceived by the observer: the shift is to higher frequencies when the source approaches and to lower frequencies when it recedes.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.