Vacuum is the term that physicists use to describe space without matter in it.
This does not mean that the space in empty; it contains energy. Quantum mechanics describes the vacuum state as the lowest energy level of a field or potential, or a displaced mirror image of matter (the opposite-signed energy to matter). The exact nature of this field is rarely examined beyond the simplest possible mathematical abstraction, and is often relegated to be an inconvenient but necessary counter-balancing quantity to the stuff theorists are really interested in. Relatively little attention is paid to the vacuum itself, but we can infer some properties and effects using our mechanism.
Philosophers have speculated about the nature of the vacuum, in an attempt to understand effects that cannot be otherwise explained. Over the years, the vacuum has been described as: an invisible medium, a fizzing fount of energy, a flowing aether, a collection of invisible particles, a fluctuating cloud that surrounds matter, or just nothing (void).
Here's a quick digest of our ideas of the subject of the vacuum.
Our definition of vacuum is different again, but can be used to describe standard phenomena associated with the vacuum. First, we do not begin with a background space (3D space is derived from the interactions of many entities having 1-dimensional structure). Instead, we have void, which is filled with waves, which are structured in pairs as bosons.
The bosons propagate by advancing their phase, and they couple with other bosons when they are both in the Quantization Condition (QC). This phase advancement implicitly generates time, and where sufficient interactions occur, space emerges.
In this picture, space itself is not a fundamental physical thing; instead, space is a generalised and simplified expression of the time intervals at which multiple events occur. Space is abstract, and it can be useful to use this abstraction as a stage (background) for things to fill it, but that would give you the wrong ideas about what is fundamental. You can define a point in this abstract space and, while accepting that the point has no physical meaning, imagine counting the bosons that propagate past it. This leads us to the simplest definition of vacuum that we can offer: it is the flux of bosons, which are the remnants of fermions radiated from elsewhere (see physicality).
QFT is based on uniqueness of scenarios/states, with the vacuum having its own state. However, we don't regard the vacuum as being a single state, e.g. a field value for a point or domain.
Instead, we see the vacuum as many bosons, so our picture of uniqueness is different from the QM view. In terms of degeneracy, our vacuum is almost unaffected by the new radiation of a fermion, firstly because it radiates away from the locality (unless it is confined), and secondly, the vacuum contains so many bosons, that one boson hardly makes a difference to its statistics. Thus, we can safely say that our vacuum is degenerate in the human environment. In the case of black holes, degeneracy is not a safe assumption.
Likewise, a particle can interact with the vacuum without affecting the statistics, and the bosons are usually independent (lacking a coherent source where multiple bosons have a relevant context).
This mechanism offers a reasonable explanation for the Casimir effect, a result of ‘quantum fluctuations’, which we understand to be the direct fermion solutions arising from bosons of the vacuum flux.
When two conducting plates are presented near to each other, the vacuum contribution results in a small 'electromotive force' as the massive conserved components' positions are occasionally displaced, resulting in a change in the mean position and velocity of their particles. Depending on the configuration of the flux, this can manifest as repulsion where the external flux causes solutions to occur further away from the centre of the experiment, along with a small occlusion of the flux by the other plate that prevents an equal contribution from the 'inside'. Alternativelty, there will be an 'attractive' component, where the flux is directed [sic] parallel to the plates, and any interactions with the conserved fermions will result in an electromotive force that brings the plates closer together.
We need further work to quantify all these components, and illustrate which is dominant in various configurations.
Now for some fun. Here are our speculations on what would happen if the vacuum flux were to be suddenly removed:
Of course, this couldn't happen in reality. However, we define the vacuum as being the uncollapsed quanta of former fermions, so we think it is very likely that there is a natural variation in the vacuum flux across large distances, depending on the near-local concentrations of matter, and radiation from events in the wider environment.