Radial velocity refers to the speed at which an object moves toward or away from an observer. It is commonly used in astronomy to measure the motion of stars and planets relative to Earth. This measurement is crucial for understanding the dynamics of celestial bodies and can indicate the presence of exoplanets. Astronomers often use the Doppler effect to determine radial velocity. When a star moves toward us, its light shifts to shorter wavelengths, appearing bluer; when it moves away, the light shifts to longer wavelengths, appearing redder. This technique helps in studying the motion of objects like stars, galaxies, and exoplanets.