Cosmology
In physical cosmology, the term peculiar velocity (or peculiar motion) refers to the components of a receding galaxy's velocity that cannot be explained by Hubble's law.
According to Hubble, and as verified by many astronomers, a galaxy is receding from us at a speed proportional to its distance. The relationship between speed and distance would be exact in the absence of other effects.
Galaxies are not distributed evenly throughout observable space, but typically found in groups or clusters, ranging in size from fewer than a dozen to several thousands. All these nearby galaxies have a gravitational effect, to the extent that the original galaxy can have a velocity of over 1,000 km/s in an apparently random direction. This velocity will therefore add, or subtract, from the radial velocity that one would expect from Hubble's law.
The main consequence is that, in determining the distance of a single galaxy, a possible error must be assumed. This error becomes smaller, relative to the total speed, as the distance increases.
A more accurate estimate can be made by taking the average velocity of a group of galaxies: the peculiar velocities, assumed to be essentially random, will cancel each other, leaving a much more accurate measurement.
Read more about this topic: Peculiar Velocity