ABM—acronym for antiballistic missile, a defensive weapon designed to intercept attacking ballistic missiles.

ABM Treaty—an agreement between the United States and Russia that prohibited each country from developing or deploying more missiles at sea, in outer space, or on mobile equipment on land. The accord was signed and implemented in 1972, but the U.S. said recently it will withdraw from the treaty this June so it can build a missile defense system.

ballistic missile—a self-propelled weapon that first uses a rocket motor, then gravity to reach its target. When the rocket burns out in flight, the trajectory—or arc—of the rocket is determined by gravity. It is able to reach targets far away with a high degree of accuracy.

CIA—acronym for the Central Intelligence Agency, one of several organizations responsible for gathering and evaluating foreign intelligence information vital to the security of the U.S.

Cold War—the post-World War II struggle between the United States and its allies, and the former Union of Soviet Socialist Republics (USSR) and its allies. The period lasted from the mid-1940s to the end of the 1980s.

cruise missile—a small aircraft with no pilot, armed with either a conventional or nuclear warhead. It's powered by a jet engine and has its own guidance system, computer, and radar for keeping course. Its range is up to 2,000 miles, and it can travel 500 mph and fly as low as 50 to 350 feet, or 5 to 35 stories high in the air.

deterrence—prevention of war by threat. This is the core strategy of U.S. nuclear weapon policy. By building and deploying thousands of nuclear warheads, the goal is to make other countries think twice before firing a nuclear weapon at America.

first-strike weapons—systems that have the accuracy and power to destroy an adversary's nuclear weapons and its ability to retaliate.

Hiroshima—the city in Japan where the first atomic bomb was dropped, on August 6, 1945, during World War II (1939-1945). According to U.S. estimates, 60,000 to 70,000 people were killed or missing as a result of the bomb, and many more were made homeless. (In 1940, the population of Hiroshima had been 343,698.) The blast also destroyed more than 4 square miles of the city, completely destroying 68 percent of Hiroshima's buildings; another 24 percent were damaged. The U.S. bombed Nagasaki, another Japanese city, on August 9. Nagasaki was the last nuclear bomb used against a civilian population.

ICBM—acronym for intercontinental ballistic missile. These missiles can travel thousands of miles and reach their targets with great accuracy in 30 minutes.

NATO—an acronym for the North Atlantic Treaty Organization, which was established in 1949 by representatives from 12 nations. During the Cold War, Western European nations felt too weak to defend themselves individually against a potential Soviet attack. In 1947, they got together to form a structure for cooperation in their defense. The North Atlantic Treaty, which was signed in Washington, D.C., on April 4, 1949, provided for mutual defense and collective security, primarily against the threat of aggression by the USSR. The original signers included Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom, and the United States. Greece and Turkey joined in 1952, the Federal Republic of Germany in 1955, Spain in 1982 and Switzerland in 2002. It was the first peacetime alliance joined by the United States.

nuclear weapons—explosive devices designed to release nuclear energy on a large scale, used primarily in military applications. The first atomic bomb (or A-bomb), which was tested on July 16, 1945, at Alamogordo, New Mexico, represented a completely new type of explosive.

second-strike weapons—weapons capable of surviving an adversary's first attack. Fired from submarines, for example, they lack the accuracy to destroy individual targets such as missile silos, but instead are aimed at less precise targets, such as cities.

SALT I and SALT II—Strategic Arms Limitation Talks. These were two rounds of negotiations that began in November 1969 between the United States and the USSR on the regulation of the nuclear arms competition between the two countries. SALT I talks ended in January 1972, resulting in two agreements: the Antiballistic Missile Treaty, and the Interim Agreement on the Limitation of Strategic Offensive Arms. SALT II talks began in September 1972 and ended in January 1979. The United States signed a SALT II agreement with the USSR, but never ratified it.

START—acronym for Strategic Arms Reduction Talks. When arms control talks resumed between the superpowers in 1982, President Reagan gave them this new name. START I, an agreement signed in 1991, required both countries to reduce their strategic nuclear arsenal by about 25 percent. Each side also agreed to cut conventional weapons and to continue withdrawing forces in stages from Europe.

SDI—short for Strategic Defense Initiative. This was the U.S. military research program for making an antiballistic missile (ABM) defense system a reality. First proposed by President Ronald Reagan in March 1983, the goal of the system was eventually to protect the United States from nuclear attack. SDI was canceled by President Clinton in 1993, after $50 billion had been spent and years of research and testing failed to make much progress.

Star Wars—Not THAT Star Wars! Although named after the popular 1977 movie by George Lucas, this Star Wars refers to President Reagan's SDI program, which had a space-based weapons component.