What is a plural in medical terms?

I am not sure if you're referring to 'pleural', which refers to the lining between the lungs and the muscles of the thorax. The pleural space is the space around the lungs within the cavity of the thorax and it contains pleural fluid, which helps reduce friction of breathing as well as other things. A person can also have a pleural effusion or pleurisy, which are both conditions affecting that space.