Liminal space. The Threshold. The world between the veils, the crossroads, the real world.
The word Limen is latin for "threshold."
Liminal space is the perception of reality inclusive of a creature's subplane of origin, but exclusive of all others. For example, a naiad comes from the subplane of water. A naiad, has the potential to dwell in both the liminal world and in her own subplane of water at the same time. She perceives all creatures that dwell in her subplane and all creatures that are present in liminal space, regardless of elemental origin. The nixie in question would not be able to see into the subplane of air, but in the liminal space a naiad could trade goods and services with a wisp, which comes from the subplane of air. A naiad could also make war on any other creature appearing in liminal space.
Every subplane is layered around liminal space. All animals, including humans, are native to liminal space, as are all nonmagical plants. liminal space is known as the real world. nothing built in a subplane can be seen by anything other than a member of that subplane, and to go into a building that does not exist in liminal space is to be completely absent from the liminal. For this reason, elementals tend to build their elemental buildings in the same space as human buildings. Ancient humans also used to construct ruins out in the wilderness for elementals to come and go away from human controlled areas.
Most elementals can choose at will to be present in liminal space or they can be summoned with elemental magic. There are notable exceptions:
familiars -- sorcerer's familiars or wizard's familiars
summoned or enslaved elementals
elemental war parties
The umbral subplane, being a construct of human wizards and magi, works a bit differently than the natural subplanes in relation to liminal space. Magical things, plants, and animals cannot be seen from the umbral subplane. while humans in the umbral subplane still dwell in liminal space, they appear to elementals as indistinct shadows, and can pass through and around them just as if they were from a natural subplane.
liminal space is inclusive of the universe and all of reality.