Utopia, n. "no place" (from Greek ou, 'not' + topos, 'place').
Uchronia, n. "no time" (from Greek ou, 'not' + khronos, 'time')
The word 'uchronia' was introduced in 1876, in Charles Renouvier's novel Uchronie (L'Utopie dans l'histoire), esquisse historique apocryphe du développement de la civilisation européenne tel qu'il n'a pas été, tel qu'il aurait pu être,1 a story of an alternate history in which Roman emperor Marcus Aurelius was succeeded by philosopher Avidius Cassius. Constantine never comes into power, and Christianity never gains a foothold.
Today it is common to use uchronia as a synonym for 'alternate history'2. However, it is also used in two other senses. In his book Predictionary, Mikhail Epstein defined uchronia as a time of stagnation: "As soon as utopia finds its fulfillment in history, it turns into uchronia, a disruption of history itself." Although Predictionary is just another collection of imaginary words, this definition has gained a foothold after being absorbed into Merriam-Webster's list of new words and slang3 and, perhaps more relevantly, Urban Dictionary.
Wikipedia, meanwhile, defines uchronia as a type of science fiction/fantasy setting which is generally consistent with our world, aside from the presence of magic, but which is 'out of time', generally an unspecified period between the bronze age and the Renaissance. Arguably Tolkian's Middle Earth was this sort of setting, as was Robert E. Howard's Hyborian Age and many of Lloyd Alexander's children's books.
1. Or, in English, Uchronia (Utopia in History), an Apocryphal Sketch of the Development of European Civilization Not as It Was But as It Might Have Been.
2. A practice greatly influenced by the alternate history booklist site http://www.uchronia.net/.
3. Currently found immediately below Ubuntu.