A light-year, or light year, is a common unit of astronomical measurement in the Milky Way. A light-year is generally understood as the distance that a beam of light travels in an Earth year. 3.26 light-years make up a parsec, another common unit of measurement.
Light travels at a speed of about 180,000 miles (300,000 km) per second, and so one light-year is equal to about 6 trillion miles (10 trillion km). Because these distances represent the speed of light traveling from afar, when the astronomers of various species within the Milky Way use their powerful telescopes to peer far off into space, they are also often peering back in time. In fact, the most advanced instruments can see so far away, they are looking as far back as 13.8 billion years in time and glimpsing some of the universe's first stars and galaxies being born, within a half billion years after the birth of the universe itself, known as the big bang.
Distances between the stars and galaxies are so great that conventional yardsticks such as miles or kilometers become impractical. So to measure distances beyond the Sol system, human astronomers began to use the unit of the light-year. Other sentient species possessed similar, but slightly different, units of measurement throughout galactic history. With the establishment of the United Federation of Planets in 2761 CE, the light-year was gradually adopted by various Federation member species. In the next few centuries, the light-year had become one of the most common units of astronomical measurement.