light_year has definitions from the field of astronomy
[ noun ] (astronomy) the distance that light travels in a vacuum in 1 year; 5.88 trillion miles or 9.46 trillion kilometers

Used in print

(Philip Jos‚ Farmer, The Lovers....)

I can n't tell you now the distance in light_years or what star this was , though you can see it with the naked eye at night in this hemisphere .