An Etymological Dictionary of Astronomy and Astrophysics
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1) A distance in → comoving coordinates between two points in space at a given cosmological time. In other words, the distance between two nearby objects in the Universe which remains constant with epoch if the two objects are moving with the → Hubble flow. More specifically, it is the → proper distance divided by the ratio of the → scale factor of the Universe between then, a(t)_em, and now, a(t)_obs: D_C = D_proper . [a(t)_obs/a(t)_em]. In terms of → redshift (z), it is the proper distance multiplied by (1 + z). At the present epoch, i.e. a = a(t_obs) = 1, D_C = D_proper. If the objects have no peculiar velocity their comoving distance at any time is the same as their distance today.
The comoving distance of the → cosmic horizon is about 48 × 10⁹→ light-years.

2) Transverse comoving distance: In a non-flat Universe, the comoving distance between two events at the same → redshift but separated on the sky by some angle. It is expressed by trigonometric functions of → curvature, → comoving distance, and the → Hubble distance accounting for the curvature of space. In a flat universe (Ω_k) it is the same as the → comoving distance.

3) Line-of-sight comoving distance: The total line-of-sight comoving distance from us to a distant object computed by integrating the infinitesimal comoving distance contributions between nearby events along the radial ray from the time t_emit, when the light from the object was emitted, to the time t_obs, when the object is observed.

→ comoving; → distance.