Redshift is the phenomenon in physics and astronomy where the observed wavelength of electromagnetic radiation from a distant source is shifted toward the longer wavelengths, specifically the red end of the visible spectrum. This observed shift is fundamentally linked to the Doppler effect, meaning that as a source moves away from an observer, the observed frequencies of light are lowered. While the redshift can be attributed to the source’s radial velocity relative to the observer, in the context of large-scale cosmology, the observed redshift of distant galaxies is interpreted as evidence of the expansion of space itself. By measuring the degree of redshift, astrophysicists can determine the relative speed and estimated distance of celestial objects, providing crucial data for understanding the evolution and geometry of the observable universe.