The period of time following a war.
Although this is a general expression that could apply to any war, it is mostly used about the years following World War II. The actual time span changes with the context, it is usually applied when phenomena regarding the aftermath of the war was in play in the area discussed.
In its most strict sense it covers the late 40's and early 50's, and describes a period of tension, anti-communism in the West, anti-capitalism in the East and the rebuilding of Europe and Japan. More often than not it stretches well into the 60's, as the same factors where at play then.
From the 70's and on, we've been back in the pre-war state.