A light-year (abbreviation: l.y.), sometimes written light year or lightyear, is a unit of length used informally to express astronomical distances. it is approximately 10 trillion kilometres (or about 6 trillion miles).one trillion here is taken to be 1012 (one million million). as defined by the international astronomical union (iau), a light-year is the distance that light travels in vacuum in one julian year. because it includes the word year, the term light-year is sometimes misinterpreted as a unit of time. the light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. the unit usually used in professional astrometry is the parsec (symbol: pc, approximately 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).