A light-year, alternatively spelled light year, is a unit of length used to express astronomical distances and is equal to exactly 9460730472580.8 km, which is approximately 9.46 trillion kilometres or 5.88 trillion miles. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year. Despite its inclusion of the word "year", the term is a unit of distance, not a unit of time.