light-year
(l.y.) A unit of distance equal to the distance traveled through space in one year by light, radio waves, or any other form of electromagnetic radiation. Since all electromagnetic radiation travels in a vacuum at the speed of light, one light-year equals about 9.4605 × 1012 km. Distances expressed in light-years give the time that radiation would take to cross that distance. One light-year equals 0.3066 parsecs, 63 240 astronomical units, or a parallax of 3.259 arc seconds. Analogous but smaller units of distance, such as the light-month, light-week, light-day, and light-second are also used, often to indicate the size of an object (such as the core of an active galaxy) whose output is varying: the timescale of the variations imposes an upper limit on the size, the conditions being unlikely to change more quickly than the time it takes for light to travel across the region.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
light-year
[′līt ‚yir] (astrophysics)
A unit of measurement of astronomical distance; it is the distance light travels in one sidereal year and is equivalent to 9.461 × 1012 kilometers or 5.879 × 1012 miles.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.