uchronia
An idealized or fictional conception of a particular period of time, especially in the past.
countable, uncountable
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 causally if not immediately to the reader's time.
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.
countable, uncountable
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.
Where it comes from
Borrowed from French uchronie, formed after utopie (“utopia”), from Ancient Greek οὐ (ou, “not”) + Ancient Greek χρόνος (khrónos, “time”) + -ia.
Adapted from the Wiktionary entry uchronia (CC BY-SA 4.0).