A formula relating the observed redshift of an extragalactic object to its distance in the expanding Universe. For redshifts of about 0.1 or less, this relationship is linear and appears as a straight line on the Hubble diagram. The relationship does not hold for objects so close together that their mutual gravitational attraction overrides the expansion of the Universe (e.g. galaxies in a cluster), nor for galaxies very close to us which may even show a blueshift. For redshifts much larger than 0.1, the relation becomes non-linear, and depends in a complicated way on cosmological parameters such as the curvature of spacetime.