Light-year
The distance light travels in one year, about 9.5 trillion kilometers, used to measure the gulf between the stars.
A light-year is a distance, not a length of time: it is how far light travels in a year, roughly 9.5 trillion kilometers, or 63,000 times the distance from Earth to the Sun. Because light is the fastest thing there is, the light-year is a convenient yardstick for the vast gaps between stars, where counting in kilometers becomes unwieldy.
It also means you are always looking into the past. Light from a star 100 light-years away left it a century ago, so you see the star as it was then. The nearest star to the Sun, Proxima Centauri, is about 4.2 light-years off: close in cosmic terms, yet still tens of thousands of years away for any craft we can build. gravityfinder turns every catalog distance into light-years, and into a travel time you can actually feel.