uchronia

noun2

1. •
   An imaginary setting of a work of fiction derived from assuming that a single particular real-world event had occurred differently than it did, causing history to differ from then on; alternate timeline.

   • For Portelli, uchronia comprises 'that amazing scene in which the author imagines what would have happened if a certain historical event had not taken place'. — 2007
2. •
   An idealized or fictional conception of a particular period of time, especially in the past.

   • This uchronia actually exerted some influence in its time, converting many readers to socialism because they wanted to live in the world of Bellamy's vision. — 2011