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, 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 (though this may change due to other, local factors, such as the motion of a galaxy within a cluster).