sun·set 
(sŭnsĕt′)
(sŭnsĕt′)n.
1. The event or time of the disappearance of the upper circumferential edge of the sun as it sets below the horizon.
2. The sky as the sun sets: a rosy sunset.
3. A decline or final phase: the sunset of an empire.
4. Law The automatic expiration of a statutory provision on a previously established date, in the absence of reauthorization: The law's sunset was July 1.
adj.
Law Providing for an automatic expiration.
v. sun·set·ted, sun·set·ting, sun·sets
Law
Law
v.intr.
To expire on a previously established date, as a statutory provision.
v.tr.
To provide for the expiration of (a program or agency) by means of a sunset provision.
[Adj. and v., on the model of SUNSHINE (as in sunshine law).]