The time taken for light from a distant object to reach the Earth; sometimes also known as light travel time. Light from nearby galaxies takes several million years to reach us, but the lookback time to very distant galaxies and quasars may be of the same order as the Hubble time. Galaxies which are so far away that their lookback time would be greater than the age of the Universe can never be observed.

http://www.astro.ucla.edu/~wright/CosmoCalc.html Online calculator for converting redshift to lookback time and other parameters.