Peculiar Velocity is the motion of an object with respect to local rest. "Rest" depends upon observation and purpose. For a star within a Galaxy, "rest" might be taken as the mean motion of nearby stars. For distant galaxies, e.g., in Cosmology studies, "rest" might be taken as the motion respect to the velocity determined by Hubble's Law, i.e., per Hubble Constant (H0).
Sunyaev-Zel'dovich Effect (SZ Effect)