SDI, which stands for Strategic Defense Initiative, was a space-based missile defense program initiated by the United States government in the 1980s. While the term SDI is widely known and used, there are also several synonyms for it that can be used in certain contexts. Some of these include National Missile Defense, Ballistic Missile Defense, Ground-based Midcourse Defense, and Boost-phase Intercept. Each of these terms describes a specific type of missile defense strategy or technology that aims to protect against potential attacks by ballistic missiles. While the use of these terms may differ depending on the situation, they all share the common goal of protecting against the threat of missile attacks.