Define Light year ?

Hint:The light year is a unit of length used to express astronomical distances and to measure distance between planets, galaxies etc.A light year is a measurement of distance. A light year is the distance a beam of light travels in one earth year. It is denoted by ‘ly’. One light year is about trillion miles $(9.46$ trillion km$)$.

Complete answer:
When we use telescopes to look at distant objects present in the galaxy, we are actually looking back in time. Light travels at a speed of $3\times {{10}^{8}}m{{s}^{-1}}$ or $3\times {{10}^{5}}km{{s}^{-1}}$. The objects in space are so far that it takes a lot of time for their light to reach us. The farther the object is the farther in the past we see. The sun is about $93$miles from earth and the sun's light takes about $8.3$ minutes to reach us. This means that we always see the sun as it was 8.3 minutes ago.

The value of light year in SI unit is equivalent to:
$9.4607\times {{10}^{15}}\,m=9.4607\,Pm$
$(1Pm$ or petameter$={{10}^{15}}m)$
Light year is the distance travelled by light in a vacuum in one Julian year is equal to $365.25$ days. Therefore, one light year $=365.25$ days.
One light year in the astronomical unit$=63241\,au$
The closest star is Orion Nebula which is at a distance of $7,861,000,000,000,000$ miles. In light years it is $1300$ light years. Therefore, measuring the astronomical distances in light years helps astronomers to determine how far back in time they are viewing.

Note: Since light takes time to travel to our eyes which means that everything we view in the sky during nights has already appeared. The nearest star to our sun is Proxima Centauri which is $4.2$ light years away.Sometimes astronomers use a unit parsec as an alternative to light years.