Divergence

 refers to the point at which the Fallout setting has separated from the real world timeline, becoming the retro-futuristic setting we all know and love. For the implications of the divergence, see the Fallout setting and Fallout world articles.

The exact point of divergence has been intentionally left vague, though it is known that it has separated sometime after World War II. However, one can argue that the divergence happened sporadically throughout history, e.g. in the several cases of Zetan abductions, or existence of the ancient city of Ubar. And also distinctly human things like corporations existing before divergence, such as The Whitespring (instead of the Greenbrier) being constructed in 1858 and the Sunset Sarsaparilla Company being founded in 1918. That said, the divergence did not mean that the United States developed across completely different lines: For example, the 1960s counter-culture movement that popularized the term "Hippie" did occur. In addition, real world failures became successes in-universe, such as the Pancor Jackhammer and the continuation of the USSR.