What is a Light Year?

Light travels at 186,212 miles per second, or, 300,000 kilometers, if you prefer. A light year is the distance which light will travel in one terrestrial year's time. It is not quite 5,880,000,000,000 miles, or well over 9,460,000,000,000 kilometers.

Everything Setanta said is correct. From Wolfram I see a light year is also almost a third of a parsec (I never knew that before!)
http://www.wolframalpha.com/input/?i=light+year

It's the distance that light travels in a year.

(Set's answer is more detailed, and correct).

Thanks everyone! :-)

Must add this: some young people assume that a light year must be a measurement of time because of the word "year." It's not. It's a measure of distance.

Just out of interest, does sound travel at the same speed as light? Is there such a thing as a "sound year" then?

1) Sound doesn't travel in space. It requires a gas (i.e. air) a liquid (i.e. water) or a solid to travel.

2) Sound travels at different speeds depending on what medium it is traveling in (i.e. it travels at a different speed in water than in air... in fact even the temperature will affect its speed).

3) Sound travels much much slower than light in any medium. This is why you see lightening before you hear the thunder, the light-- traveling at 186,000 miles every second gets to your eyes almost instantly. The sound travels a little more than 10 miles per second... it gets their much more slowly.

The distance which sound travels in any given period of time is dependent upon the medium through which it travels. Light has both wave and particle properties, and can travel through the (relative) vacuum of "outer space." Sound can only travel through a conducting medium which is a gas, a liquid or a solid. At sea level on the earth, sound travels through the ambient atmosphere at a rate of 1100 feet per second (obviously, it is much, much slower than light). This can be used practically. If you see a flash of lightening, and note the time, in seconds, which elapse before you hear the thunder, you can get a good estimate of the distance you were from the lightening--five seconds roughly equals one mile, and three seconds roughly equals one kilometer.

Had to read through those replies a couple of times before I could make sense of them but it's very very interesting. Thank you very much guys.

a light year is the distance between your last sip from the thermal 160z travel coffee mug and the next rest stop.

he he he. When my daughter asked me this morning, my first thought was "there is no such thing, only on Star Trek" .

A light year is a duration for those who detest getting old...

But a Lightyear is to infinity and beyond.....

Interesting we are around 8 light minutes away from the sun, 1 1/2 light second from the moon and a few light hours to the outer planets.

A light year is a long distance indeed in relationship to our solar system.

All true, but since we are being official, a light year is the distance light travels in one year in a vacuum. The speed of light is also dependent on the medium through which it travels. Light travels more slowly through the fiber optic line carrying this reply than in a vacuum for example.

Yes light travel speed is cut almost in half if memory serve me correctly and that is why light will bend on hitting water for example and the blue light in nuclear reactors pools is cause by this slowing down if I remember correctly.

Jeeze, everything's political with you guys . . . now you have to bring up what is found between a politician's ears.

1 light year = 1.89210568 × 1024 beardseconds