Light Year

Distance that light travels in a year through a vacuumSpace entirely devoid of matter (called also, by way of distinction, absolute vacuum). In a more general sense, a space, as the interior of a closed vessel, which has been evacuated to a high degree by a pump or other artificial means.. This unit is derived from the fact that light (electromagnetic radiation) takes a finite length of time to travel through space. Defined as 299,792,458 m/s and often rounded to 300 x 10⁶ m/s.