A medianworld is a logical extrapolation of an individual's personality into "what the world would be like if the median person were me, and the normal distribution of personalities was translated through all relevant dimensions to center on that".
Dath Ilan is the medianworld of Eliezer Yudkowsky.
Medianworlds can have differently-shaped tech trees from the individual's real world, as well as different customs and governmental structures.
A related concept is the Author Aliens world, where the physical features of the individuals in a civilization are modified to reflect the individual's psychology too.