Light year is the unit of

The correct answer is distance.

A light-year is a unit of astronomical distance rather than time, despite containing the word "year". It is defined as the distance that light travels in a vacuum in one Julian year (365.25 days).

To understand this concept, we can use the basic formula for distance:
$\text{Distance}=\text{Speed}\times \text{Time}$
where the speed is the speed of light in a vacuum and the time is one year.

Let's calculate the approximate value of one light-year in meters:
1. The speed of light in a vacuum ($c$) is approximately:
$c\approx 3\times {10}^8 \text{m/s}$
2. One year in seconds ($t$) is:
$t=365.25\times 24\times 60\times 60 \text{seconds}\approx 3.156\times {10}^7 \text{s}$
3. Multiplying speed and time gives the distance of one light-year (ly):
$1 \text{ly}\approx (3\times {10}^8 \text{m/s})\times (3.156\times {10}^7 \text{s})\approx 9.46\times {10}^{15} \text{meters}$
Since it measures the spatial path traversed by light, it is a unit of distance.