Uchronia refers to a hypothetical time period of our world, in contrast to fictional lands or worlds. A concept similar to Alternative history but different in the manner that uchronic times are not easily defined (mainly placed in some distant point near prehistory), and the result reminds of a conworld.
The word is a portmanteau of the word utopia (Greek u-topos not-land), replacing topos with chronos (time).
Tolkien's Middle-earth, and Howard's Hyborean Age are examples of uchronian periods, set roughly in the prehistoric times, around and after Atlantis.
External links
www.uchronia.net