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Definition of "light-year"

The distance travelled by a beam of light in one year. The light-year is used when expressing the huge distances between stars and galaxies. Since there are several different definitions of the year, the light-year is not a strictly defined constant. Usually, however, the adopted value of the light-year is the distance travelled by light in a Julian year of 365.25 days. In terms of other units, this makes the light-year equivalent to;

63 241astronomical units (AU)
9 460 730 000 000km
5 878 625 000 000miles