Coined by the theoretician Jakob Steen Madsen, the word denotes the presence of an absolute vacuum.
Interpretational variations on the phenomenon exist.
Stephen Goodfellow interprets subspace on an interstellar magnitude, as a spherical volume in space sustained by a rotating (see dynamo effect) magnetically repulsive high energy plasma. The resulting subspace volume induces gravity (gravity without a corresponding quantity of mass,) causing space to implode on the subspace, thus an equilibrium is maintained. Possible matter manufacture (as per virtual particle production,) and time dilation at the boundary may have further implications.
Goodfellow’s subspace solar model from “Can Gravity be Induced?”