Light Year
A light year is the distance that light travels in a year (about 6 trillion miles). It is a measurement of distance, not time.
One cool aspect of measuring distances like this, is that - the farther into space you look, the farther back in time you will see. It takes light so long to reach us that when you finally see the light from that object, you are seeing how that object looked when that light left it. Consider this - the light coming in through the window today - that light left the Sun 8 minutes ago. You are seeing the Sun as it appeared 8 minutes ago.
So, in a real sense, a telescope is a time machine. Confused? Watch this cool clip from last year's Cosmos reboot.
So, in a real sense, a telescope is a time machine. Confused? Watch this cool clip from last year's Cosmos reboot.