[MHGinTN]

Essentially, the term refers to the enormous energy field from which virtual particles pop in and out of our spacetime coordinate system. The name derives, IIRC, from the fact that even if a cubicle yard of empty space were chilled to absolute zero this energy would still be seething and virtual particles popping in and out of our spacetime. If you like, think of it as the latent energy of spacetime expansion, like a rubber band being stretched.

[dynachrome]

Thanks.