Containment was the policy of the United States after World War II to assist small countries in their resistance against communism. The idea was to counter act the Soviet pressures so that communism would not spread further.
The term was coined by diplomat George F. Kennan in 1946, in an anonymous article written to the journal Foreign Affairs, titled The Sources of Soviet Conduct. Kennan argued that the Soviet Union suffered from paranoia and fear of foreign invasion, and thus would seek to build a network of buffer states. However, Soviet leaders were also inherently risk averse and would quickly back down when confronted with an American show of force.
Containment was a major part of the Truman Doctrine, laid out in 1947 and first applied in Greece and Turkey.