Definitions for "Lightyear"
The distance light travels in a year in a vacuum at the rate of 300.000 km/s is defined as a lightyear. ly = 9.460.530 × 106 km = 63.239,74 AU
The distance that light travels in one year - about 9.5 million, million kilometers. Once we start talking about distances to stars, it gets silly to talk about them in kilometres. So a new measurement is used called a lightyear. If a star is 4 lightyears away, it would take 4 years to travel there at the speed of light.
One lightyear is the distance traveled by light in vacuum in a period of 1 year, and equals almost 6000 billion miles. The quantities lighthour, lightminute and lightsecond are defined similarly.