Radial velocity refers to the speed at which an object moves toward or away from an observer. In astronomy, it is often used to measure the motion of stars and galaxies relative to Earth. This measurement is crucial for understanding the dynamics of celestial bodies and can provide insights into their distances and compositions. Astronomers typically determine radial velocity by observing the Doppler effect, which causes the light from moving objects to shift in wavelength. When an object moves toward us, its light appears blue-shifted, while light from an object moving away appears red-shifted. This technique is essential for discovering exoplanets and studying the expansion of the universe.