What does "Doppler shift" refer to in the context of frequency and velocity?

"Doppler shift" refers to the change in frequency of a wave in relation to an observer moving relative to the source of the wave. This phenomenon occurs when either the source of the frequency or the observer is in motion. When the source approaches the observer, the waves are compressed, leading to a higher frequency (or a "blue shift"). Conversely, when the source moves away from the observer, the waves are stretched, resulting in a lower frequency (or a "red shift"). This effect is observed in various wave types, including sound and electromagnetic waves.

The other options do not accurately describe the Doppler shift. A consistent frequency, regardless of motion, does not account for the variations experienced due to movement between the source and observer. Measuring frequency accurately at rest does not involve the principles of the Doppler shift, which specifically revolves around relative motion. Lastly, while the Doppler effect does apply to sound waves, it is not exclusive to them; it also applies to electromagnetic waves, such as light. Thus, the definition of Doppler shift is most accurately captured by the change in frequency perceived due to the motion of the source or observer.