Added 10 October 2026 light-year noun Categories: Describing Words noun ! Definition from WordNet 3.1, Princeton University. More about light-year at Wordnik → Where our words come from → The distance that light travels in a vacuum in 1 year; 5.88 trillion miles or 9.46 trillion kilometers. Related Words: lightyearmegalightyearkilolightyear Back to today's word