Light Year:

A light-year is the distance traveled by light moving in a vacuum in the course of one year, at its accepted velocity of 299,792,458 meters per second (186,282 miles per second). A light-year equals about 9.46053x1012 km (5.878x1012 miles), or 63,240 astronomical units. About 3.262 light-years equal one parsec.

Excerpt from the Encyclopedia Britannica without permission.