When astronomers cite the "Visible Universe", what exactly do they mean?

The big bang was 12 billion  years ago so the left hand side of the universe has been travelling left for 12Billion years and the right hand side has been travelling right for 12 billion years they are now 24 billion years away from each other. They of course have not been separating at the speed of light but they are more than 12 billion years apart. Light does travel at the speed of light :-} so we can only see light that has travelled 12 billion light years some light from the furthest reaches of the expanding universe has not yet reached us, hence ''visible universe''

The pertinent question: is there universe other than the observable universe? Should dark energy, dark matter and parallel universes be talked about as distinct objects, when they are by definition concepts that are unmeasurable? Fields are ethereal objects, but there are defined procedures for measuring their magnitude and intensity; these procedures provide a direct mapping between the symbolic representations of our minds, allowing verification with respect whatever these fields represent. Whenever we fail to distinguish what happens inside our minds from what occurs outside our minds, we confuse the symbolic reality of our mind, with that which the symbols represent. When a single member of a group believes in a non-existent thing, he is labelled deluded, but when most members of the group believe in the same nothing, then the sceptic is censured by the group. In instances where the group reificates a symbol without defining any procedure by which the symbol may be measured; there is no possibility of  verification. The symbol has now taken on an existence of its own, independent of any basis outside of our minds. ---------- To illustrate this issue with a less abstract thought experiment: If a photon were to move toward a non rotating black hole; would the photon, as seen from our perspective, accelerate indefinitely? At some point, it would appear, the photon must cross some horizon, an ultimate bound to our observable universe. This bound of maximum possible acceleration, it would seem, provides an ultimate limit to our observable universe. Is there any validity to the idea that this photon has any verifiable existence within our universe, once this bound has been crossed? What is the meaning of any such existence?