Wikipedia
The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly metres (just over miles).
Just as the second forms the basis for other units of time, the light-second can form the basis for other units of length, ranging from the light-nanosecond (just under one international foot) to the light-minute, light-hour and light-day, which are sometimes used in popular science publications. The more commonly used light-year is also presently defined to be equal to precisely light-seconds, since the definition of a year is based on a Julian year (not Gregorian year) of exactly 365.25 days, each of exactly SI seconds.