Definitions

from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun The distance that light travels in a vacuum in one year, approximately 9.46 trillion kilometers (5.88 trillion miles).
  • noun Informal A long way.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun astronomy A unit of length (abbreviation ly; equal to just under 10 trillion kilometres (1016 metres)) equal to the distance light travels in one Julian year; used to measure extremely large distances.
  • noun informal A very long way.
  • noun informal, nonstandard A very long time.

Etymologies

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  • (Symbol: ly) A unit of measurement of length, equal to just under ten trillion kilometres. The distance that light travels in a vacuum in one Julian year.

    Exactly 9,460,730,472,580.8 km

    About 5,878,625,373,183.61 statute miles

    (Wikipedia)

    May 29, 2008