from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. change in the apparent frequency of a wave as observer and source move toward or away from each other
From Johann Christian Andreas Doppler, the scientist who first proposed this effect. (Wiktionary)