The space between the visceral pleura and the parietal pleura. In the healthy individual, this space is occupied by just a few millilitres of lubricating fluid (over the entire lung!) and is thus a potential space rather than a true space.
The pleural space, in the disease state, can be filled with all sorts of stuff, resulting in: haemothorax, pneumothorax, chylothorax, pleural effusion and empyema.