Empyema: Empyema is a condition in which pus and fluid from infected tissue collects in a body cavity. The name comes from the Greek word empyein meaning pus-producing (suppurate). Empyema is most often used to refer to collections of pus in the space around the lungs (pleural cavity), but sometimes refers to similar collections in the gall bladder or the pelvic cavity. Empyema may have a number of causes but is most frequently a complication of pneumonia. Its development can be divided into three phases: an acute phase in which the body cavity fills with a thin fluid containing some pus; a second stage in which the fluid thickens and a fibrous, coagulation protein (fibrin) begins to accumulate within the cavity, and a third or chronic stage in which the lung or other organ is encased within a thick covering of fibrous material.