The term systemic velocity is used for diffuse objects such as galaxies (especially), disks, or clouds, to indicate the average velocity of all the portions, since different portions of the object have different movements. In other words, it is the velocity of the entire system. It might literally be determined by averaging, but if the object has a discernible center, that can be of use. The term is used in situations where the velocities of portions of the object are of interest, e.g., considering the orbits of a galaxy's stars.
For distant galaxies or galaxy clusters, the systemic velocity would be the sum of the peculiar velocity and the consequence of the Hubble expansion.
The systemic redshift is the redshift associated with an object's systemic velocity.