In astronomy, the redshift parameter z is defined as the fractional change in wavelength. It compares the wavelength observed from an astronomical source with the wavelength measured in the source’s rest frame: z = (observed wavelength − emitted wavelength) / emitted wavelength.
A positive z means the observed wavelength is longer, shifting spectral features toward the red end of the spectrum. A negative value indicates a blueshift, in which the wavelength becomes shorter. Because z is a ratio, it has no units; a value of 1 means the observed wavelength is twice the emitted wavelength.
Astronomers determine redshift by identifying familiar spectral lines from elements such as hydrogen and comparing their observed positions with laboratory values. Redshift can result from relative motion, the expansion of the universe, or gravity. Frequency is related inversely to wavelength, so it also changes, but the parameter z is conventionally defined using wavelength. At high speeds, converting redshift directly into velocity requires relativistic treatment.