Definition: A sunset rule is a provision included in a legislative bill that limits the effects of the bill. It does this by specifying an ending date after which certain aspects or benefits of the bill will no longer apply.
The most well-known sunset rule in recent memory, is the one attached to the Economic Growth and Tax Relief Reconciliation Act (EGTRRA) of 2001. It was under this act that Section 529 plans gained significant tax advantages, and estate taxes were eliminated for calendar year 2010.
In 2006, Congress moved to make Section 529 plan benefits permanent, exempting them from this sunset rule. However, no action was taken on the temporary elimination of estate taxes, leading most people to conclude that estate taxes will again reappear in 2011.