SDI stands for Strategic Defense Initiative.
The SDI, which was first proposed by U.S. President Ronald Reagan in 1983, was a U.S. Government initiative to develop highly sophisticated anti-ballistic missile systems. Because of the science fiction-like nature of some of the proposals, it was given the nickname Star Wars. The SDI triggered both domestic and international opposition, especially from the Soviet Union, who were seen as the chief nuclear threat to the U.S.A. in the early 1980s.
Although the US Defense Department established a special SDI unit in 1984, subsequent U.S. administrations changed the name of the unit, and modified the original proposals.