What describes the change in frequency or wavelength of a wave as observed by someone moving relative to the wave source?

The phenomenon described is known as the Doppler effect. This effect occurs when there is a relative motion between a wave source and an observer. When the wave source moves toward the observer, the waves are compressed, leading to a higher frequency (or shorter wavelength); conversely, when the source moves away from the observer, the waves are stretched, resulting in a lower frequency (or longer wavelength). The Doppler effect can be observed in various types of waves, including sound and light waves, and is commonly experienced in everyday situations, such as the changing pitch of a passing siren.

In contrast, wave propagation refers to how waves travel through a medium and does not encompass the changes in frequency or wavelength relative to an observer. Signal attenuation describes the reduction in strength of a signal as it travels through a medium and is unrelated to frequency changes. Lastly, a phase shift pertains to a change in the phase of a wave but does not involve any change in frequency or wavelength. Thus, the Doppler effect is the term that accurately captures the relationship between frequency or wavelength changes and relative movement between the source and observer.