In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects. Proper distance roughly corresponds to where a distant object would be at a specific moment of cosmological time, measured using a long series of rulers stretched out from our position to the object's position at that time, and which can change over time due to the expansion of the universe. Comoving distance factors out the expansion of the universe, giving a distance that does not change in time due to the expansion of space. Comoving distance and proper distance are defined to be equal at the present time. That is, the two distances only differ at times other than the time at which they are measured: The universe's expansion results in the proper distance changing, while the comoving distance is unchanged by this expansion.
Read more about Comoving Distance: Comoving Coordinates, Comoving Distance and Proper Distance, Proper Distance Vs. Comoving Distance From Small Galaxies To Galaxy Clusters
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—Ralph Waldo Emerson (1803–1882)