define light-year. define light-year. the distance that light can travel in 1 year, which is approximately 149.6 trillion kilometers. the distance that light travels in 1 year, which is about 9.46 trillion kilometers. the distance that earth travels around the sun in 1 year, which is about 9.46 million kilometers. the average distance between earth and the sun, which is about 9.46 billion kilometers. the distance that light can travel in 1 year, which is approximately 149.6 billion kilometers.