The constant of proportionality (designated H) between recession velocity and distance in the Hubble law. It is a constant of proportionality but not a constant in time, because it can change over the history of the universe.
In terms of the scale factor, the Hubble constant is defined
i.e., the rate of change of the scale factor with respect to time divided by the scale factor.
Measuring the Hubble constant is difficult and remains and important task for astronomers. Present best values lie between approximately 50 km/sec/Mpc and 100 km/sec/Mpc, with a value around 70 km/sec/Mpc favored.