From The Collaborative International Dictionary of English v.0.48 [gcide]:
1. The distance over which light can travel in a year's time in a vacuum, 5.88 trillion miles (9.46 trillion kilometers); -- used as a unit in expressing stellar distances. It is more than 63,000 times as great as the distance from the earth to the sun. The star nearest to the earth's sun is about 4 light years distant. [Webster 1913 Suppl. +PJC]
2. a very large difference; -- used figuratively to express great differences (of distance, quality, time) in comparisons. "Although having the same rank and salary, they were light-years apart in their ability to perform their jobs." [PJC]
From WordNet (r) 3.0 (2006) [wn]:
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