horizon distance(particle horizon) The total distance that a light signal could have traveled since the beginning of the Universe, and hence the maximum distance that can be seen in a cosmological model.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
horizon distance[hə′rīz·ən ¦dis·təns]
The distance, at any given azimuth, to the point on the earth's surface constituting the horizon for some specified observer.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.