1. The observed shift of the characteristic spectral lines of, for example, a galaxy, toward the red, longer-wavelength, end of the spectrum as a result of the galaxy's recession from the earth.
2. The lengthening of the wavelengths of light from an object as a result of the object's motion away from the earth.

It is an example of the Doppler effect. The red shift in light from galaxies is evidence that the universe is expanding.