The time of day when the sun disappears below the western horizon.

The changes in color of the sky at sunset.

The final period of the life of a person or thing.
one's sunset years

Having a set termination date.
The tax increase legislation included a sunset clause requiring renewal to prevent the tax increase from expiring.

The region where the sun sets; the west.

To phase out.
We'll be sunsetting version 1.9 of the software shortly after releasing version 2.0 next quarter.