The Aegis Ballistic Missile Defense System (Aegis BMD or ABMD) is a United States Department of Defense Missile Defense Agency program developed to provide missile defense against short to intermediate-range ballistic missiles. It is part of the United States national missile defense strategy. Aegis BMD (also known as Sea-Based Midcourse) is designed to intercept ballistic missiles post-boost phase and prior to reentry.

An anti-ballistic missile (ABM) is a surface-to-air missile designed to counter ballistic missiles (see missile defense). Ballistic missiles are used to deliver nuclear, chemical, biological or conventional warheads in a ballistic flight trajectory. The term "anti-ballistic missile" is commonly used for systems designed to counter intercontinental ballistic missiles (ICBMs).