The rest wavelength of a source of electromagnetic radiation is the wavelength as observed if at rest in relation to the radiation source. If source and observer are in relative motion, the wavelength observed is termed the observed wavelength. Redshift (z) is calculated as:
difference in wavelength z = —————————————————————————— rest wavelength observed wavelength - rest wavelength z = ————————————————————————————————————— rest wavelength or observed wavelength rest wavelength = ——————————————————— 1 + Z
Given the Hubble expansion (the increasing the distance between objects), our meaning of "at rest" here specifically does not mean "at rest within local the Hubble flow", but that the distance between source and observer is not changing, i.e., they are at rest relative to each other.