Final answer:
The Cold War is described as the ideological and geopolitical struggle between the United States and the Soviet Union, marked by an arms race, proxy wars, and the threat of mutually assured destruction, without escalating into direct combat.
Step-by-step explanation:
The main word or concept that best describes the Cold War is the ideological and geopolitical struggle for supremacy that transpired globally between the United States and the Soviet Union post-World War II. This era of history was characterized by nuclear deterrence, espionage, technological competition like the Space Race, political influence across different nations, and proxy wars, but never escalated into direct large-scale combat between the two superpowers. The Cold War represents a significant period in the 20th century that shaped international relations and global politics, with the presence of an "iron curtain" dividing Europe and the threat of mutually assured destruction looming over the world.