In physics and astronomy, redshift is an observed increase in the wavelength and decrease in the frequency of electromagnetic radiation received by a detector compared to that emitted by the source. For visible light, red is the color with the longest wavelength, so colors experiencing redshift shift towards the red part of the electromagnetic spectrum. The phenomenon goes by the same name even if it occurs at non-optical wavelengths (in fact, longer-wavelength radiation "redshifts" away from red). The corresponding shift to shorter wavelengths is called blueshift.

Astrophysical redshift occurs, for example, when a light source moves away from an observer in an analogous but not equivalent fashion to a Doppler shift of sound waves from a receding object. Redshift is used as a diagnostic in spectroscopic astrophysics to determine information about the dynamics and kinematics of distant objects. Most famously, redshifts are observed in the spectra from distant galaxies, quasars, and intergalactic gas clouds to increase proportionally with the distance to the object. Astronomers consider this to be one of the major forms of evidence that the universe is expanding, supporting the Big Bang model.