Z

Definition: Z: The ratio of the observed change in wavelength of light emitted by a moving object to the rest wavelength of the emitted light. See Doppler Effect. This ratio is related to the velocity of the object. In general, with v = velocity of the object, c is the speed of light, lambda is the rest wavelength, and delta-lambda is the observed change in the wavelength, z is given by:

z = (delta-lambda)/lamda = (sqrt(1+v/c) / sqrt(1-v/c)) - 1.
If the velocity of the object is small compared to the speed of light, then

z = (delta-lambda)/lamda = v/c
Objects at the furthest reaches of the known universe have values of z = 5 or slightly greater.