By Hidaya Aliouche, B.Sc.Reviewed by Emily Henderson, B.Sc. Empyema is defined as a collection of pus in the pleural space and is a form of purulent (infected), pleural effusion. Pleural effusion refers to the build-up of excess fluid between the layers of the pleura outside the lungs. Image Credit: ALIOUI MA/Shutterstock.com Defining the pleura, serous membrane, and...