ABL: (1) Airborne Laser. (2) Armored Box Launcher.

AEGIS: The Navy's advanced, fast reaction, high firepower, shipboard anti-air warfare
area defense system (Note: Aegis is the Greek word for "shield").

Anti-Ballistic Missile (ABM) Treaty: The ABM Treaty, which was signed by the United States and the former Soviet Union on May 26, 1972, and entered into force on October 3, 1972, constrained strategic missile defenses to a total of 200 launchers and interceptors per country- 100 at each of two widely separated deployment areas. These restrictions were intended to prevent the establishment of a nationwide defense or the creation of a base for deploying such a defense. The treaty was modified in 1974, reducing the number of ABM deployment areas permitted each side from two to one and the number of ABM launchers and interceptors from 200 to 100. However, on June 13, 2002, the United States officially withdrew from the ABM Treaty in order to pursue the development of missile defenses that would have been banned by this agreement.

Ballistic missile: A missile that travels, or releases a payload that travels, to its target after being launched and at a velocity such that it will follow a flight trajectory to a desired point. Part of the flight of longer-range ballistic missiles may occur outside the atmosphere and involve the "reentry" of the missile. Missiles that are not ballistic are aerodynamic, operating in the atmosphere.  Aerodynamic missiles include cruise and air-to-air missiles.

Boost-phase: That part of the ballistic missile flight path that begins at launch and lasts up to five minutes for a primitive liquid-fueled inter-continental ballistic missile (ICBM) or as little as 80 seconds for an advanced solid-fueled ICBM. During boost phase, the booster and sustainer engines operate, and warheads have not yet been deployed.

DSP: A system of non-imaging infrared satellites in geo-stationary orbits, fixed and mobile ground processing stations, one multi-purpose facility, and a ground communications network (GCN). DSP’s primary mission is to provide warning and limited attack assessment of a ballistic missile attack.

EWR: Early Warning Radar located in Alaska and other U.S. locations.

Ground Based Interceptor (GBI): The missile intercept of the proposed U.S. Ballistic Missile Defense (BMDS) System, the GBI will intercept incoming ballistic missile warheads outside the earth's atmosphere (exo-atmospheric) and collide with the incoming ballistic missile, thereby destroying the missile. The NMD Battle Management, Command, Control, and Communications (BMC3) will transmit information on the location of the incoming missile to the GBI during its flight. The GBI would consist of a multi-stage solid propellant booster and an exo-atmospheric kill vehicle.

Intercontinental ballistic missile (ICBM): Ballistic missiles with ranges greater than 5,500 kilometers (3,400 miles).

KEI: Kinetic Energy Intercept.

Land-based missile systems: Missile systems located on land usually in hardened bunkers and underground silos or on mobile launchers, which are more vulnerable to first-strike attacks. The mobile land-based missile systems are less vulnerable to first-strike because the positions of the missiles can be changed.

LPAR: Large Phased Array Radar.

Layered BMDS: The current and planned integrated U.S. Ballistic Missile Defense System consists of several sets of defensive interceptors that operate against incoming ballistic missiles at different phases in the missile's trajectory. Thus, there could be a first layer (e.g., boost phase) of defense with remaining targets passed on to succeeding layers (e.g., midcourse and terminal). The Bush Administration uses the terms "BMDS" in lieu of the "TMD" and "NMD" phrases that were preferred by the Clinton Administration.

Medium Extended Air Defense System (MEADS): A lightweight, highly transportable, low-to-medium altitude air defense and theater missile defense system designed to protect critical fixed and theater missile defense system designed to protect critical fixed and mobile targets.

Mid-phase (or midcourse phase): That part of the ballistic missile's trajectory, after the boost phase, when the re-entry vehicle and warhead travel freely through space outside the atmosphere. For an ICBM, this stage lasts about 20 minutes.

MKV:  Multiple Kill Vehicle.  An experimental concept that would put multiple guided kill vehicles on a single interceptor booster, allowing each to distroy multiple objects.

National missile defense (NMD): A ground-based anti-ballistic missile system designed to protect a country against ballistic missile threats. The proposed U.S. system consists of four elements: ground-based interceptors (GBI); a ground-based radar (GBR); a battle management command, control, and communications (BM/C 3) system; and a constellation of Space and Missile Tracking System (SMTS) satellites. The term was used by the Clinton Administration to differentiate systems able to intercept long-range missiles from systems able to intercept only short-range, or "theater"-range missiles. The Bush Administration integrated the concept into the larger BMDS'.

PAC (1): PATRIOT Advanced Capability. (2) Program Assessment Center. (MDA)

PAC-2/-3: PATRIOT Advanced Capability, Level 2/Level 3. Formerly called ERINT.

PAC-3: PATRIOT Advanced Capability-3.

SBIRS: Space Based Infrared System.

SBX: Sea-based X-band Radar – A moveable platform for the BMDS test bed.

STSS: Space Tracking and Surveillance System. STSS is a constellation of low-earth orbiting satellites that will detect and provide critical tracking information about ballistic missiles globally. The program is employing a capabilities-based, block upgrade acquisition approach. The first block, termed Block 06 by the Missile Defense Agency (MDA), will be deployed in 2007 to support the missile defense testbed. STSS is designed to support the BMDS which will negate missiles/warheads in boost, mid-course, and terminal phases of flight. STSS will provide tracking through all three phases; discriminate between warheads and decoys; transmit data to other systems that will be used to cue radars and provide intercept handovers; and provide data for intercept hit/kill assessments. STSS provides unique characteristics including global coverage with no foreign basing issues, continual watch on emerging and known threats, and dual phenomenology for discrimination to ensure efficient use of interceptors.

SM-2: Standard Missile-2. (U.S. Navy)

SM-3: Standard Missile-3. ( Developed by the Missile Defense Agency and the U.S. Navy in cooperation with the Japanese Defense Agency.)

SPY-1: AEGIS Radar.

Terminal-phase: The final phase of a warhead's trajectory when it re-enters the earth's atmosphere and strikes the target.

Theater High Altitude Air Defense (THAAD): The U.S. Army's air defense program designed to provide extended defense and to engage an incoming missile at ranges of up to several hundred kilometers. THAAD will deploy a hit-to-kill interceptor equipped with an infrared seeker. Unlike fragmentation warheads that explode near an object in order to destroy it, the THAAD interceptor is designed to collide with the target ballistic missile. The interception is intended to occur outside the earth's atmosphere, or high in the atmosphere.

Theater missile defense (TMD): Missile interceptors designed to destroy shorter-range ballistic missiles aimed at deployed troops or overseas facilities. Because the ABM Treaty prohibited NMD, but permitted defenses against shorter-range missiles, the Clinton Administration sought to separate TMD and NMD. The Bush Administration eliminated the distinction between NMD and TMD and incorporated both programs into the BMDS.

Theater missile: Short-range delivery system (missile) with a range of 1,000 kilometers or less.

USAF: United States Air Force