Potential space

In anatomy, a potential space is any anatomic space that is potential, as in a space that could become a real space in certain physiologic or pathophysiologic events. It is thus a space or cavity that can exist between two adjacent structures that are not tightly adjoined but usually does not open up during normal functioning. 

An example would be the pericardial space between the two layers of the pericardium. The layers of pericardium are usually next to each other separated only by a thin layer of serous fluid. However, in certain pathological conditions (e.g. pericardial effusion) the two layers of pericardium can separate, forced apart by a pathological or traumatic accumulation of fluid.