Empyema: The presence of pus in the pleural space which is between the outer surface of the lung and the chest wall. Empyema is often a complication of pneumonia caused by bacteria such as Streptococcus pneumonia, Staphylococcus aureus, or Haemophilus influenza (H. flu) type b.

The formation of the empyema is conventionally divided into three phases: exudative, fibrinopurulent and organizing. During the exudative phase, the pus accumulates. This is followed by the fibrinopurulent phase in which there is loculation of the pleural fluid (the creation of grapelike pockets of pus). In the final organizing phase there is the potential for lung entrapment by scarring.

The Greek philosopher, Aristotle, recognized empyema and described the drainage of pus with incision and a metal tube as early as 300 BC. The word "empyema" is Greek. It comes from "pyon" meaning pus.