A light-year is an astronomical unit of distance equal to 9.5 trillion kilometers (5.9 trillion miles), defined as the distance light travels in an Earth year.
The universe is big and the distances between the objects in it can be surprisingly large too. A light-year is defined as the the distance light can travel in one Earth year. It can be calculated by multiplying the speed of light (3 x 108 m/s or 300,000,000 m/s) by the length of a year (365.25 days or 3.15 x 107 seconds). Light years to meters: 9.45 x 1015 m, or 9.45 trillion kilometers!
An advantage to using the light-year as a unit of distance is that it makes us realise that light doesn’t move instantaneously. If a galaxy is a million light-years away, it means that the light has taken a million years to reach Earth. This means that when we observe a galaxy that is a million light years away, we are actually looking at the galaxy a million years ago.
For distances within a star system, the light-year is normally too big a distance to use. For example, the distance from the Sun to Earth is 0.00000156 light-years. The unit is mostly suited to intergalactic distances. Astronomers prefer to use the astronomical distance unit the parsec, but the light-year has survived as a popular unit. The term 'light-year' has been mentioned in many popular science fiction works, such as Star Trek.
This term is often confused as a unit of time because it includes the word 'year'. Sometimes people will write things like “technology is light-years ahead”; this can add to confusion about the term. A light-year is a unit of distance, like miles or kilometers—only on a much larger scale.