uchronias

Noun

uchronia (countable and uncountable, plural uchronias)

1. An idealized or fictional conception of a particular period of time, especially in the past.
   • Mercier's resort to uchronia, on the other hand, initiates a new paradigm for utopian literature not only by setting action in a specific future chronologically connected to our past and present but even more crucially by characterizing that future as one belonging to progress and thus linked...1987, Paul K. Alkon, Origins of Futuristic Fiction, page 127
2. 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.
   • Alternate or alternative histories—uchronias or counterfactuals, to employ yet two other terms—have gone through a boom-and-bust cycle lately, accruing both merits and demerits. […] S.M. Stirling falls into none of these pitfalls. In fact, his new book is simply the best uchronia in years.2002 January, Paul Di Filippo, “The Peshawar Lancers”, in SyFy, archived from the original on 14 Jun 2002