What is a light year?

A light-year is a unit of length used to express astronomical distances. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Earth year (365.25 days), which is equivalent to about 5.88 trillion miles (5.88 × 10¹²mi) or 9.46 trillion kilometres (9.46 × 10¹² km).