from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A change in the observed frequency of a wave, as of sound or light, occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart. The motion of the source causes a real shift in frequency of the wave, while the motion of the observer produces only an apparent shift in frequency. Also called Doppler shift.
After Christian Johann Doppler.(American Heritage® Dictionary of the English Language, Fourth Edition)