The term for the change in frequency of a wave in relation to an observer is known as the Doppler effect. This phenomenon occurs when there is relative motion between a wave source and an observer, leading to an observed change in the frequency of the waves. When the source approaches the observer, the frequency increases, resulting in a higher pitch in sound waves or a shift toward the blue end of the spectrum in light waves. Conversely, when the source moves away, the frequency decreases, causing a lower pitch or redshift in light. This effect has various applications in science and technology.