(Latin: res, thing)
Strictly speaking includes the whole objective order of things; it comprises both things actually existing and things capable of existing.
It is synonymous with being in the substantial sense.
The term, however, is usually employed in philosophy to designate the physical and metaphysical order of things as opposed to the conceptual or logical order called the "Ideal."
The realist as opposed to the idealist affirms that we know of the existence actual and potential of things outside of our minds, i.e., that objects have, and can have, an existence distinct from the intentional or representative existence which they have in our mental processes.
New Catholic Dictionary