The effect that describes the change in frequency or wavelength of a wave due to the motion of the observer relative to the source is known as the Doppler effect. This phenomenon occurs in various wave types, including sound and light. When the source of the wave is moving towards the observer, the frequency appears to increase (blue shift), and when it moves away, the frequency decreases (red shift). The Doppler effect is crucial in fields such as astronomy, as it helps in determining the motion of stars and galaxies, and in various technologies including radar and medical imaging.