A light-year is a unit of measurement used in astronomy to represent the distance that light travels in one year. It is approximately 9.461 trillion kilometers or 5.878 trillion miles. Since light travels at a speed of about 299,792 kilometers per second (or about 186,282 miles per second) in a vacuum, a light-year is a significant distance in the vastness of space. It is often used to describe the distances between stars and galaxies.