Of
legislation, but possibly also in
contracts any clause that imposes some
time limit on the implementation of the conditions of the
legislation.
For example, let's say the government agrees to fund the biotechnology sector through tax incentives. Usually, the government would not do so indefinitely -- the legislation granting these incentives would only apply for a certain time frame, say 5 years. The clause in the legislation that specified that the tax incentives would end in five years is a sunset clause.
This makes it inherently different from most other legislation, since most legislation is generally intended to apply until amended by another bill.