A Doppler effect refers to the change in frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave. It is commonly observed with sound waves, where an approaching source (like a siren) sounds higher in pitch, while a receding source sounds lower. This phenomenon also applies to electromagnetic waves, such as light, where it can result in a redshift or blueshift depending on the direction of movement. The Doppler effect is widely used in various fields, including astronomy, radar technology, and medical imaging.