A lacuna (pl. lacunae) is a temporary pocket dimension. When the void between realities is repeatedly breached (such as via magic portal), a lacuna can form.
Lacunae range from "featureless plain" to Lovecraftian bastardizations of spatial relations. They pick up attributes of their parent realities: if a lacuna forms between a world where the sky is blue and another where it is red, the lacuna may very well have a purple sky.
Any given lacuna will cease to exist almost immediately upon the exit of its last interworld visitor.
Reality
Lacunae exist primarily as an anything-goes setting for casual Moonburnt roleplay. By their nature, they can be pretty much any sort of setting a player may want, with no need to maintain continuity between one lacuna and another.
Artemisia has a particular knack for shaping lacunae. Between a lacuna's variability and her specific reality-warping powers, Missy can be very powerful, given the right lacuna and a human in search of a drink. The Moonburnt tag 
location: the bar refers to a recurring (nameless) bar that Missy tends to set up in lacunae, given a human patron whose ideas she can build off of.